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Alhazen's Theory of Vision and Its Reception in the WestAuthor(s): David C. LindbergSource: Isis, Vol. 58, No. 3 (Autumn, 1967), pp. 321-341Published by: on behalf ofThe University of Chicago Press The History of Science SocietyStable URL: http://www.jstor.org/stable/227990Accessed: 26-07-2015 05:13 UTC
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8/20/2019 Lindberg - Al Hazen’s Theory of Vision and Its Reception in the West (1967)
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Alhazen s
heory
o
i s i o n
n d t s
Reception
n
t h e W e st
By
David C.
Lindberg*
I
HE
MOST
SERIOUS
problem facing
the Muslim heirs
of
Greek
thought
was
the
extraordinary diversity
of their inheritance.
Among
theories of
optics,
for
instance,
Muslim
thinkers had the
following
choice:
the
emission
theory
of
sight
of
Euclid and
Ptolemy,
which
postulated
visual
rays
emanat-
ing
from the
observer's
eye;
the older
Epicurean
intromission
theory,
which
reversed the
rays
and
made them
corporeal;
the
combined
emission-intromis-
sion
theories
of
Plato
and
Galen;
and
some
enigmatic
statements
of
Aristotle
about
light
as
qualitative
change
in a
medium.1 These
Greek theories
gener-
ated a
wide assortment of
optical
theories
in
Islam,
two
of
which came
to
dominate. Hunain ibn Ishaq (d. 877), the most prolific translator of scientific
works
into
Arabic,
argued
for
a combined emission-intromission
theory
in
the
tradition
of Plato
and Galen.2 Al-Kindi
(d.
c.
873)
agreed
with Hunain
that
rays
are emitted
by
both the
visible
object
and
the
eye, although
he
couched
his
theory
in
terms of a
general
emanation
of
power
having
Stoic
and
Neoplatonic
origins
and
appropriated
the
geometrical
approach
to
optics
appearing
in
the works of
Euclid and
Ptolemy.3
Avicenna
(Ibn
Sina,
d.
1037)
took
exception
to the
views
of
Hunain
and
al-Kindi,
denying
that visual
rays
are
of
any
use in
explaining
the
process
of
sight
and
insisting
on
a
complete
intromission
theory.4
These and other Muslim
philosophers
made
important
*
University
of
Wisconsin.
A
short
version
of
this
paper
was
presented
before a
joint
meet-
ing
of the American
Association
for
the
Ad-
vancement
of
Science
and the
History
of
Science
Society,
December
1966.
The
paper
is a
product
of research
supported by
the
National
Science
Foundation.
1
Obviously
this
attempt
at classification ob-
scures
a host
of
distinctions. On Greek
optics
see
Arthur
Erich
Haas,
"Antike
Lichttheorien,"
Archiv
fiir
Geschichte der
Philosophie,
1907,
20:345-386;
J.
Hirschberg,
"Die
Optik
der alten
Griechen,"
Zeitschrift
fiir
Psychologie
und
Physiologie
der
Sinnesorgane,
1898,
16:321-351;
Albert
Lejeune,
Euclide et
Ptolemee
(Louvain:
Bibliotheque
de
l'Universite,
1948);
Lejeune,
Recherches sur
la
catoptrique grecque (Brus-
sels:
Palais de
Academies,
1957).
2
The Book
of
the
Ten Treatises
of
the
Eye,
ascribed to Hunain ibn
Ishdq
(809-877
A.D.),
trans.
Max
Meyerhof
(Cairo:
Government
Press,
1928),
pp.
31-39.
3
Graziella Federici
Vescovini,
Studi
sulla
prospettiva
medievale
(Turin:
G.
Giappichelli,
1965),
Ch.
3;
Alkindi,
De
aspectibus,
in A.
A.
Bjornbo
and
Sebastian
Vogl,
"Alkindi,
Tideus
und
Pseudo-Euklid,
Drei
optische
Werke,"
Abhandlung zur Geschichte der mathemati-
schen
Wissenschaften,
1912, 26,
Pt.
3.
4
Eilhard
Wiedemann,
"Ibn
Sina's An-
schauung
vom
Sehvorgang,"
Archiv
fiir
die Ge-
321
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DAVID C. LINDBERG
contributions
through
their
criticisms of
Greek theories and
their
syntheses
of
disparate
elements of Greek
thought;
moreover,
their
influence on Western
optical
thought
was
far from
negligible.
Yet,
none of them
created an in-
clusive optical system to rival that of Ptolemy; they dealt with but one or
another
aspect
of
sight,
usually
in
the
space
of a
few
paragraphs
or a
few
pages.
The first
comprehensive
and
systematic
alternative to Greek
optical
theo-
ries was
formulated
by
Alhazen
(Ibn
al-Haitham,
d.
c.
1039),
a
figure
of
im-
mense
importance
in
the
history
of
optics.
Alhazen
leveled a
devastating
attack
at
prevailing
optical
theories and formulated a
grand
and
viable
alter-
native. Moreover, he had a
profound
influence on
the West:
his
principal
work on
optics
(Kitab
al-manazir,
cited
by Western
authors
as
De
aspectibus
or
Perspectiva)
5
was
translated
into
Latin late
in
the
twelfth
or
early
in the
thirteenth
century
and
dominated Western
optical thought
until
early
in
the
seventeenth
century.
Modern
optical
thought
issues,
by
direct
descent,
from the
work of Alhazen
and his immediate
followers.
The central feature of Alhazen's
system
is
its
theory
of direct
vision,
and
with
this
topic
the
Perspectiva
opens.6
Alhazen
notes, first,
the
effect
of
bright
lights
on the
eye.
"We
find,"
he
says,
"that when the
eye
looks
into
exceed-
ingly
bright lights,
it suffers
greatly
because of them and
is
injured.
For when
an
observer looks
at the
body
of the
sun,
he
cannot see it
well,
since
his
eye
suffers
pain
because
of
the
light."
7
Clearly
this
implies
an
action of
bright
bodies on the
eye,
for
injury
is
something
inflicted
by
an
agent
on
a
recipient
and could not, in the case of the eye, result from emission of the eye's own
ray.
The
phenomenon
of
the
afterimage
supports
the same
position:
schichte
der
Naturwissenschaften
und der
Technik, 1913,
4:239-241.
This
is
a
German
translation
of a short
work on
physics by
Avi-
cenna.
Avicenna
expresses
similar
views in
many
other
works.
5
The
only printed
edition
of
this work is
Opticae
thesaurus
Alhazeni
Arabis
libri
septem,
nunc
primum
editi a
Federico Risnero
(Basel,
1572);
the title
Opticae
thesaurus
was
given
to
the book
by
Risner.
Manuscripts
of the Arabic
text have
recently
been discovered
(see
Max
Krause,
"Stambuler Handschriften
islamischer
Mathematiker,"
Quellen
und Studien
zur
Geschichte
der
Mathematik,
Astronomie
und
Physik,
1936,
Abt.
B, 3:476,
which
lists
five lo-
cated
in
Istanbul);
an
edition is
being
pre-
pared
by
A. I. Sabra.
My
study
has
been
limited
to the
Latin
text;
but it is
abundantly
clear-
from
a
comparison
of
English
translations
of
the
same
section made
from both
Latin and
Arabic
texts
(Stephen
L.
Polyak,
The Retina
[Chicago:
Univ.
Chicago
Press,
1941], pp.
109-
111),
from
the
recent
study by
Matthias
Schramm (Ibn al-Haythams Weg zur Physik
[Wiesbaden:
Franz
Steiner,
1963]),
and
from
a
comparison
of
the
Opticae
thesaurus with Al-
hazen's shorter tracts
on
optics
translated
from
Arabic
by
Wiedemann, Baarman, Winter,
and
others-that
the
Latin text
faithfully
repro-
duces the
substance of
Alhazen's
ideas. If it
can
be shown that there are
significant
differ-
ences
between
the
Arabic and
Latin
texts,
then
I
must be content with
elucidating
the
Latin
tradition,
which was influential
in
the
West.
I
have
repeatedly
checked the
Risner text
against
earlier
Latin
manuscripts
(British
Museum
Royal
MS
12.G.VII
and
Bruges
MS
512)
and
find no
differences in
substance.
The
best
secondary
works on Alhazen's
op-
tics are Vasco Ronchi, Histoire de la lumiere,
trans.
J.
Taton
(Paris:
Librairie Armand
Colin,
1956),
a
translation of his
Storia
della
luce
(Bo-
logna:
N.
Zanichelli,
1952);
H.
J. J.
Winter,
"The
Optical
Researches of
Ibn
al-Haitham,"
Centaurus,
1954,
3:190-210;
Vescovini,
Studi,
Ch.
7;
Schramm,
Weg
zur
Physik;
and
Leopold
Schnaase,
Die
Optik
Alhazens
(Stargard:
A.
Muller,
1889).
There
is also
a
two-volume
study
by
a
contemporary
Egyptian
physicist,
Mustafa
Nazif,
Ibn
al-Haitham:
His
Optical
Researches
and Discoveries
(Cairo:
Nuri
Press,
1942-1943).
6
I.e.,
the
Latin
text,
which seems to lack
the
first few brief chapters of the Arabic text. Cf.
Eilhard
Wiedemann,
"Zu Ibn
al-Haitams
Op-
tik,"
Arch. Gesch.
Naturw.
Tech.,
1910,
3:4.
7
Opticae
thesaurus,
I,
Sec. 1.
p.
1. All trans-
lations are rendered
from
this edition.
Space
has not
permitted
inclusion of the
Latin text.
322
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ALHAZEN'S THEORY
OF
VISION
. . .
when an
observer
looks
at
a
bright
fire
and
allows
it to
linger
in
his
vision for a
long
time,
if
he then
transfers
his
gaze
to a
weakly
illuminated
place,
he
will
[continue
to]
see
the same
thing
[i.e.,
the
brightness].
. . .
Fi-
nally this fades away and vision returns to its normal disposition.8
After
overwhelming
his reader
with
this and similar
data,
Alhazen
concludes,
"All
these
things
indicate that
light
produces
some effect
in the
eye."9
So far Alhazen
has confined
his
argument
to
light
from what
we
would
call
self-luminous
objects.
Such luminous
rays,
filtering
through
mist or
dust,
had been
recognized
since
antiquity,10
but
Alhazen
has more
in
mind than
these. He
argues
that
every
visible
object
is
seen
by
the
emission of its
own
light,
though
illumination
by
a
self-luminous
body
is
a normal
prerequisite:
It
has
been demonstrated above
that
light
issues
in
all
directions
opposite
any
body
that is illuminated with
any light.
Therefore when the
eye
is
oppo-
site
a
visible
object
and the
object
is
illuminated with
light
of
any
sort,
light
comes
to
the
surface of the
eye
from
the
light
of
the visible
object."
However,
an
observer
perceives
the color of the
visible
object
as well as
its
light.
This is a similar
process
and
always
accompanies
the
perception
of
light:
It has
been shown
already
that the
form of
color
of
any
colored
body,
illumi-
nated
by
any
light
whatsoever,
always
accompanies
the
light
emanating
from
that
body
to
any
region
opposite
the
body.
. . .
Therefore the
form of
the
color of a
visible
body always
accompanies
the
light
coming
to
the
eye
from
the
light
of the
body.
And since
light
and color come to the surface of the
eye simultaneously,
the
eye perceives
the color
of
the visible
object
on account
of the
light
coming
to it
from
the
object.
It
is
proper,
therefore,
that
the
eye
should not
perceive
the color of the visible
object except through
the form
of
color
accompanying
the
light
to
the
eye;
and
the form of color is
always
mixed
with the
form
of
light
. .12
Light
and
color,
the first
twenty-two
visible
intentions
identified
by
Al-
hazen,
are
perceived
by
sense alone without
the
support
of
any
process
of
ratiocination. The
remaining twenty
visible
intentions-including
such
things as remoteness, position, shape, magnitude, motion, rest, and beauty -
are
perceived
visually,
but
only by
processes
of
recognition,
distinction,
and
argumentation performed
by
the
virtus distinctiva.
Light
and
color
re-
main
the
primary
visible
intentions,
and the
others
are perceived
through
their mediation.13
Six conditions
must
be fulfilled if the
forms
of
light
and color
(issuing
8
Ibid.
nately; consequently,
I
have
made no
attempt
9
Ibid.
to
distinguish
between them in
my English
translation. In the
passage
quoted,
Alhazen
lOE.g.,
Galen,
De
usu
partium,
X, 12, in speaks
of
light,
but
elsewhere
(e.g., p.
14) he
(Euvres anatomiques, physiologiques et medi- speaks of the form of light
cales de
Galien,
trans. Charles
Daremberg,
Vol.
12
Ibid., p. 7.
I
(Paris:
J.-B.
Baillire,
1854),
p.
639.
13
Ibid.,
II,
Ch.
2, pp.
34 if.
(chapter
desig-
11
Opt.
thes., I,
Sec.
14,
p.
7.
Here and else-
nations
go
back
to
the
Arabic
original;
see
where,
the
Latin
text of
Alhazen's treatise
em- n.
23
below).
On the
meaning
of "intention"
ploys
the terms
lux
and
lumen
indiscrimi-
see
Vescovini,
Studi,
pp.
64-69,
80-85.
323
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DAVID C. LINDBERG
from an
object
in
all
directions)
are
to enter the
eye
of the observer and
bring
about
perception:
14
(1)
There must be a certain distance between
the
eye
and
the visible
object.
(2)
The
visible
object
must
be
directly
in
front
of
the
eye,
that
is,
within the visual field.
(3)
The
object
must be either self-
luminous
or
illuminated
by
light
from another
body. (4)
The
visible
object
must
possess
magnitude;
that
is,
lines
drawn
from
the
extremities of the
visible
object
to the
center
of the
eye
must
intercept,
on
the surface of
the
glacial
humor
(crystalline
lens),
"a
segment
of sensible
magnitude by
comparison
with
the
whole surface of
the
glacial
humor."15
(5)
The medium
or
media between the
object
and
the
observer
must
be
transparent.
(6)
The
object
must be
dense and solid.
This
final
requirement
is instructive
regard-
ing
the nature of
light.
In
the
first
place,
the
object
must be dense and solid
because
only
dense and
solid bodies have
color and
part
of
the act
of vision
is
perception
of the
color
of the
body. Secondly,
if
the
body
were not dense
and
solid
(i.e.,
if
it
were
transparent),
the
light
by
which
it
is illuminated
would
pass
through
without
opposition.
In
this
event,
there
would
be no
light
in
the
surface of the
body
capable
of
emanating
its form to
the
observer:
When
there
is a
transparent
body
opposite
the
eye
and it is
illuminated
by
light
from the direction of the
observer,
the
light
passes
through
it
and
is
not
fixed in its
surface;
and
thus in
the surface
of the
body
opposite
the
eye
there
will
be
no
light
from which a
form
can come
to
the
eye.16
Evidently
the
light
(or
the form of
light)
issuing
from
a
nonluminous
body
is
not
its
own but
has
been
deposited
there
by
an
illuminating
body.
The
foregoing
discussion
makes it clear
(1)
that the
forms
of
light
and
color
issue
in all
directions from
self-luminous or illuminated bodies
through
transparent
media and
(2)
that
the
forms
of
light
and color make
an
impres-
sion
on the
eye.
But
it
might
still be
argued
that
the forms of
light
and color
do
not
issue
from the
visible
object
unless
triggered
by
rays emanating
from
the
observer's
eye;
that
is,
one could
still claim
that
visual
rays
play
a neces-
sary
role
in
vision.
In order
to
demonstrate
the
futility
of this
hypothesis
of
visual rays, Alhazen undertakes a long and tightly knit argument. Let us
suppose,
he
says,
"that
rays
issue
from the
eye
and
pass through
the trans-
parent
body
[between
the
eye
and the
object]
to the
object
of
sight
and that
perception
occurs
by
means
of those
rays."
17
Either
these
rays
take
something
from
the
object
and
return
it
to the
eye,
or
they
do not. If
they
do
not,
the
eye
cannot
perceive
the
object
by
means of them. But
this is counter
to the
original supposition
that
rays
issue
from
the
eye
to
perceive
the
object.
Con-
sequently,
it must be concluded
that the
rays
do transmit
something
from
the
object
to the
eye:
14 Opt. thes., I, Ch. 7, pp. 22-23. World, Alhazen embraced the emission theory,
15
Ibid., I,
Sec.
40,
p.
23.
but
I
am
making
no
attempt
in
this
article
to
16
Ibid.,
Sec.
42,
p.
23.
examine the
development
of
Alhazen's
theories;
17
Ibid.,
Sec.
23,
p.
14.
Here
Alhazen assumes
cf.
Eilhard
Wiedemann,
"Zur Geschichte der
that which
he
proposes
to
disprove.
In
his Lehre vom
Sehen,"
Annalen
der
Physik
und
earlier
treatise,
On the
Configuration
of
the
Chemie,
1890,
neue
Folge,
39:473.
324
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ALHAZEN'S THEORY
OF
VISION
Therefore
[according
to
the
emission
view]
those
[visual]
rays
that
perceive
the
visible
object
transmit
something
to the
eye,
by
means
of
which
the
eye
perceives
the
object.
And
since the
rays
transmit
something
to
the
eye,
by
means of which the eye perceives the object, the eye perceives the light and
color
in
the visible
object
by
no other means than
through
something coming
to the
eye
from the
light
and color
in
the
object.
..
.18
Thus,
even
in the
emission
theory, sight
is
ultimately
achieved
by
communi-
cation
of
something
from the
object
to
the
eye.
Since it has
already
been
demonstrated that the forms of
light
and
color
emanate
in all directions
from
the
visible
object
without the
hypothesis
of visual
rays,
of what
advantage
are
the
visual
rays?
As
Alhazen
expresses
this
impressive
argument,
. .
.
sight
occurs
only
as
something
of
the visible
object
comes
from the
ob-
ject [to the eye], whether or not rays issue from the eye. Now it has already
been
declared that
sight
is
achieved
only
if
the
body
intermediate between
the
eye
and
the
visible
object
is
transparent,
and it
is not
achieved
if
the
medium is
opaque.
.
. . Since
.
. . the forms of
the
light
and
color
in
the
visible
object
reach
the
eye (if
they
were
[originally] opposite
the
eye),
that
which comes
from
the
visible
object
to the
eye (through
which
the
eye
per-
ceives
the
light
and color
in the visible
object
no
matter
what the situation
[with
respect
to visual
rays])
is
merely
that
form,
whether
or
not
rays
issue
[from
the
eye].
Furthermore,
it
has been
shown that the
forms
of
light
and
color are
always generated
in air and
in
all
transport
bodies
and
are
always
extended
to the
opposite
regions,
whether
or
not the
eye
is
present.
There-
fore the
egress
of
rays
[from
the
eye]
is
superfluous
and useless.19
Not
yet
satisfied that
he has
disposed
of
the
theory
of
visual
rays,
Alhazen
launches
a further attack. If
it
is
assumed,
once
again,
that
sight
is
due to
something issuing
from
the
eye,
either that
thing
is
body
or
it is not.
If it is
body,
it
follows
that when one
looks at
the vault
of
the
heavens,
body
flows
from the
eye
to
fill
the
entire
space
between the
heavens and
the
eye,
yet
without
destroying
or
diminishing
the
eye
in
any
way.
Since
that
is
obviously
impossible,
that which
flows
from the
eye
is not
body.
But
if
that which issues
from
the
eye
is not
body,
it cannot
perceive
the
the
object,
since
"there is
no
perception except in
bodies."20
Thus,
by
means of a reductio ad absurdum,
Alhazen has demonstrated
that a
ray
issuing
from
the
eye
cannot
be
respon-
sible for
sight;
but
he has not demonstrated
that no
ray
issues
from the
eye.
However,
he
concludes
that
if the
rays
are not
responsible
for
sight, they
are not
sensible;
therefore
they
are
conjectural,
"and
nothing ought
to be
believed
except
through
reason or
by
sight."21
Although
it
appears
that
Alhazen
has
completely
discredited the
theory
of visual
rays,
the
obscurity
of the text
has led to
recent
confusion
on
this
18
Opt.
thes., I,
Sec.
23,
p.
14.
require
that
the
ray issuing
from the
eye
be
19Ibid. Note Alhazen's appeal to the prin- material.
ciple
of
economy.
21
".
.
.
et
nihil debet
putari
nisi
per
ra-
20
Ibid. This also
excludes
the
possibility,
tionem vel a
visu"
(ibid.).
The last three words
Alhazen
thinks,
that
something
issuing
from in
this
Latin
text are not found
in
the
Risner
the
eye
could take
something
from the visible edition
but are included
in
British
Museum
object
and return it to
the
eye;
that too would
Royal
MS
12.G.VII,
fol. 7v.
325
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DAVID
C.
LINDBERG
point.22
Friedrich
Risner,
editor of
the
only
printed
edition
of Alhazen's
optics
(1572),
divided
the
work into
sections
and
gave
each
a title. Section
24
of Book I
he
entitled:
"Vision
seems
to
occur
through
avvavyetav,
that
is,
rays
simultaneously
received and emitted." 23Alhazen's text
immediately
beneath
this title
begins
as
follows:
It has been asserted
on
account
of this
[i.e.,
the
argument
of the
previous
section]
that
both schools of
thought [presumably
emission
and
intromission]
speak
the
truth
and
that both beliefs
are
correct
and
consistent;
but
one
does
not
suffice
without
the
other,
and
there can
be
no
sight except
through
that
which
is maintained
by
both schools
of
thought.24
A number
of
historians,
deceived
by
Risner's
title,
have concluded
that
in
the
opening
lines
of
Section
24
Alhazen backs
down on
his
denial
of the
existence of visual
rays.25
But,
in
fact,
Alhazen never admits the real existence
of the
rays.
He is
willing
only
to allow
mathematicians,
who
are
concerned
with
a
mathematical
account
of
the
phenomena
rather than with
the
real
nature
of
things,
to use
visual
rays
to
represent
the
geometrical properties
of
sight.
Indeed,
these
rays
or lines
are
indispensable
if
one
is to
understand
how
sight
occurs,
for
through
them one is able
to
visualize
"the nature
of
the
arrangement
according
to which
the
eye
is affected
by
the form
[of
light
or
color]."26
But,
according
to
Alhazen,
all mathematicians
who
postulate
visual
rays
"use
only imaginary
lines
in
their
demonstrations,
and
they
call
them 'radial lines.'
"27
Moreover, the belief "of those who consider radial
22
There was
no
confusion
in medieval
Europe. Roger
Bacon
(Opus
majus,
V,
1,
Dist.
7,
Ch.
3)
and
John
Pecham
(Perspectiva
commu-
nis, I,
Props.
44-46)
were
fully
aware
of Al-
hazen's
uncompromising
opposition
to visual
rays
as
agents
that
go
out
and
seize
something
from the
object
and
convey
it back
to the
eye.
23
"Visio
videtur
fieri
per
avvav,yetav,
id
est
receptos
simul et
emissos
radios"
(Opt.
thes.,
I,
Sec.
24,
p.
15).
Aetius
Placita,
IV,
13,
11
(Her-
mann
Diels,
Doxographi
Graeci
[3rd
ed.,
Berlin:
W. de Gruyter, 1958], p. 404) uses the term
-vva6vyeta
(simultaneous
radiation)
to describe
Plato's double-emission
theory
of
sight (Plato
does
not
use the
term
himself;
see
Haas,
"Licht-
theorien,"
p.
393,
n.
100),
and
it
is
clear
that
Risner
is
comparing
Alhazen's
theory
to
just
such
a double-emission
theory.
It
should be
noted that Risner
added
the
sections,
but the
division of
chapters
goes
back
to
the Arabic
original;
cf.
Risner's
edition
with
the
summary
of
chapters
in the Arabic
text
contained
in Lutfi
M.
Sa'di,
"Ibn-al-Haitham
(Alhazen),
Medieval
Scientist,"
University
of
Michigan Medical Bulletin, 1956, 22, No. 6:258-
259.
24
"Et
declaratum
est ex
hoc,
quod
duae
sec-
tae
dicant verum:
et
quod
duae
opiniones
sint
rectae
et
convenientes:
sed non
completur
al-
tera
earum,
nisi
per
alteram,
neque
potest
esse
visio,
nisi
per
illud,
quod
aggregatur
ex
duabus
sectis"
(Opt.
thes.,
I,
Sec.
24,
p. 15).
The
open-
ing phrase
is
somewhat
ambiguous,
and
the
context
is of little
help.
The
passage
could
equally
well be
translated,
"It has
been
demon-
strated
on
account
of this
. .
,"
which alters
the
meaning
subtly
but
significantly.
The
Arabic text
is
not relevant at
this
point,
since
I
am
concerned
with
the
misleading
character
of Risner's
title and the
Latin text immedi-
ately
following
it.
However,
either translation
is capable of being interpreted as an admission
by
Alhazen that
visual
rays
exist.
25
For
example,
Ronchi
states
that
Alhazen,
"apres
avoir
affirm6
nettement
que
la vision
ne
se fait
pas
au
moyen
de
rayons
emis
par
l'eil,
en
vient
a
une
sorte
de
compromis
et
avance
que
la vision
semble
se faire concur-
remment
par
des
rayons
recus
et
par
des
rayons
6mis"
(Histoire, pp.
39-40).
Following
Ronchi
and
Risner,
I
made
the
same
mistake
in
my
article
"The
Perspectiva
communis of
John
Pecham:
Its
Influence, Sources,
and
Content,"
Archives
internationales
d'histoire
des
sciences,
1965,
18:47-48.
26
Opt.
thes.,
I,
Sec.
24,
p.
15.
27
Ibid.,
Sec.
23,
p.
15.
Cf.
Galen,
De
usu
partium,
X,
12,
in
(Euvres,
trans.
Daremberg,
Vol.
I,
p.
639.
326
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ALHAZEN'S
THEORY
OF
VISION
lines
to
be
imaginary
is
true,
and the
belief
of
those
who
suppose
that some-
thing
[actually]
issues from the
eye
is
false."28
Thus
visual
rays
(or
radial
lines)
are mere
geometrical
constructions,
useful
in
demonstrating
the
proper-
ties of sight. They can serve as a mathematical hypothesis, but they have no
physical
existence.
However,
if
these
rays
are
imaginary,
why imagine
them
to issue from
the
eye
rather
than from the
visible
object?
This is
probably
a
concession,
first,
to
traditional
geometrical optics (e.g.,
the
work
of Euclid
and
Ptolemy),
which had
been
combined
with
belief
in
visual
rays;
and sec-
ond,
to
the natural
intelligibility
of a
center
of
perspective
from
which
rays
emanate
to
perceive
visible
things.
An
intromission
theory
of
vision
brings
new
urgency
to the determination
of which ocular
organ
is
the
sensitive one.
Indeed,
the
question
changes
from
"Which
organ
is the source
of
the
rays?"
to "Which
organ
receives
the
rays?";
and this
change
in
question
widens the
scope
of
permissible
answers. In the
visual
ray
theory,
there was little
alternative
to
placing
the source
of
rays
at the center
of the
eye
so
that
the
rays
would
be
unrefracted as
they emerged
and,
consequently,
capable
of
accurately
determining
the
location
of
objects
in
space.29
The intromission
theory,
however,
opens
the
question
to further
investigation.
Nevertheless,
Alhazen's conclusion
was
substantially
the
same
as that
of
antiquity.
Islamic
prohibitions
against
dissection
left Muslim in-
vestigators
with
little choice but to
rely
on
Greek
descriptions
of
the
eye
and
pronouncements
regarding
the sensitive
organ.
Alhazen's
description
of the
eye
varies
only
in minor
details
from the
descriptions
of
Galen and Rufus
of
Ephesus,
and
Alhazen
even
admits that
it
is drawn from earlier anatomical
treatises.30
He
identifies
four
tunics
(consolidativa,
uvea, cornea,
and
aranea)
and three
humors
(aqueous
humor,
vitreous
humor,
and
crystalline lens),31
and
follows
Galen
closely
in
arguing
that
the
glacial
humor
(crystalline
lens)
is
the sensitive
organ:
"If
injury
should
befall
the
glacial
humor,
the
other
tunics
remaining
sound,
sight
is
destroyed;
if the other tunics should be
corrupted,
their
transparency
and the health of the
glacial
humor
being
re-
28
Opt.
thes.,
I,
Sec.
23,
p.
15.
29
Cf.
L'Optique
de Claude
Ptolemee dans
la version latine d'apres I'arabe de l'emir Eu-
gene
de
Sicile,
ed. Albert
Lejeune
(Louvain:
Publications
Universitaires
de
Louvain,
1956),
pp.
148-149.
30
Opt.
thes.,
I, Sec.,
13,
p.
7.
Galen and
Rufus
were the
most
important
ancient
sources
on
the
anatomy
and
physiology
of the
eye,
though
Alhazen's
direct
dependence
on
them cannot be
demonstrated.
The
drawings
of
the
eye
con-
tained
in the Risner edition
(p.
6 of
Alhazen's
Opt.
thes. and
p.
87
of Witelo's
Optica,
bound
with
the
Opt. thes.)
do not
originate
with
Alha-
zen or
Witelo,
but
were taken
from
the
De
cor-
poris humani fabrica (1st ed., 1543) of Andreas
Vesalius;
cf.
J.
Hirschberg,
Geschichte der
Augenheilkunde,
in
Graefe-Saemisch
Hand-
buch der
gesamten
Augenheilkunde,
Vol.
XIII
(Leipzig:
Wilhelm
Engelmann,
1908),
p.
149.
For
a
description
and
reproductions
of
diagrams
taken
from
Arabic
manuscripts
of
Alhazen's
Kitab
al-manizir
and other Islamic
authors,
including one dated 1083 and apparently copied
by
Alhazen's
son-in-law,
see
Polyak,
Retina,
pp.
114-119
and
Figs.
7-12.
31
Galen
(2nd
half
of the
2nd
century)
iden-
tifies the same
three
humors,
but he is
a
bit
ambiguous
on
the tunics.
He
describes seven
tunics,
but
these
appear
to be circles
or
layers
rather
than tunics
in
the
usual
sense.
Cf.
Galen,
De usu
partium,
X,
2,
trans.
Daremberg,
Vol.
I,
pp.
609-614.
On
Galen's
anatomy
of
the
eye
and
theory
of
vision,
see
Hirschberg,
"Die
Optik
der alten
Griechen,"
pp.
347-351;
Polyak,
Retina,
pp.
97-101.
Rufus
of
Ephesus
(1st half of the 2nd century) describes four
tunics,
but
they
do not
correspond
exactly
to
the
four
tunics described
by
Alhazen.
On Ru-
fus,
see
iEuvres de
Rufus
d'Atphese,
ed. and
trans. Charles
Daremberg
and Ch.
Lmile Ruelle
(Paris:
J.
B.
Bailliere,
1879),
pp.
154,
170-172.
327
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DAVID C. LINDBERG
tained,
sight
is
not
destroyed."
32
Alhazen
specifies,
further,
that the
sensitive
part
of the
glacial
humor
is its
front
surface,
which
is
concentric
with
the
cornea.33
Thus far a very general account of Alhazen's theory of vision has been
presented:
the
forms of
light
and
color
emanate from
bodies
in all
directions;
they
pass through
the
transparent
cornea
of the
eye
and fall on the front
surface
of the
glacial
humor.
But enormous
obstacles
remain to
be overcome.
Chief of
these is to
show how various
parts
of the same
object
are
distinguished
from
each other.34 The
observer
perceives
not
only
the
presence
of
light
and
color,
but
particular
patterns
of
light
and color.
Clearly,
vision
is more
than
mere
reception
of
forms;
reception
occurs
in
such
a
way
that different
objects
(or
different
parts
of
the
same
object)
perceived
at the
same
time
are
perceived
as
being
distinct
and
in
their true
spatial relationship.
It
appears
as
though
this could be
explained
by
attributing
the
perception
of different
parts
of
the
object,
scattered about
the
visual
field,
to
different
parts
of
the
surface
of the
glacial
humor.
But this raises a
serious
difficulty:
from each
part
of
the
object
the forms
of
light
and color emanate in all
directions;
consequently,
every
part
of
the
glacial
humor
should receive forms of
light
and
color
from
every
part
of
the
object,
and total
confusion should
result.
Alhazen
overcame
the
difficulty by considering
the
visual
field
point
by
point.35
Every
point
on a
visible
object
radiates
the forms of its
light
and
color
in
all
directions,
but
only
the
form
directed
toward the
center
of
curva-
ture of the front surface of the eye is incident on the eye perpendicularly
and enters without
refraction.36
Shifting
to the
language
of
geometrical
lines,
Alhazen
points
out that
from
every
point
on
an
object
there are
infinitely
many
lines
incident
on
the front
of the
eye,
but
only
one
line from
each
point
is incident
perpendicularly
and
is
hence unrefracted. The
forms that
are unrefracted
are most
efficacious
in
vision,
and
refracted forms
yield
only
an indistinct
impression.
By
thus
restricting
himself to
forms or
rays
propa-
gated
rectilinearly,
Alhazen has eliminated all
possibility
of confusion in
the
eye:
there
is
a
single
ray
from
every
point
on the
object,
passing
in
a
straight
line toward
the
center
of the observer's
eye.37
Because
these
rays
are
recti-
32
Opt.
thes.,
I,
Sec.
16,
p.
8.
Cf.
Galen,
De
usu
partium,
X,
3,
trans.
Daremberg,
Vol.
I,
p.
608.
A similar
argument
is
presented
by
Hunain
ibn
Ishaq
(Ten
Treatises
of
the
Eye,
p. 4)
and
by
later authors
such
as Bacon
and
Pecham.
33".
. .
et erit
forma
ordinata,
sicut est
ordinata
in
superficie
rei
visae,
et in
parte
ista
superficiei
glacialis"
(Opt.
thes., I,
Sec.
24,
p.
15).
Cf.
ibid.,
Sec.
40,
p.
23.
"Oportet,
ut cen-
trum
superficiei glacialis
et
centrum
superficiei
visus sint
unum
punctum"
(ibid.,
Sec.
23,
p. 14).
34
In the emission theory, the eye is an
organ
with
directional
sensitivity: rays
issue
forth
in
all
directions,
and the location of
the
object
in
space
is
determined
by
the direction
of
the
ray
terminating
on
it.
In
discarding
the
emission
theory,
Alhazen
gave up
this direc-
tional
sensitivity:
the front
surface
of
the lens
is
sensitive to the
presence
of
light
but
not to
the
direction
from which
it
came.
This
gener-
ates the
problem
of
distinguishing
various
parts
of
the visual
field.
s3
Ibid.,
Sec.
18,
pp.
9-10.
36
It is
uncertain
in this
context whether
"point"
means
"very
small area"
or
"that
which has no
part."
In
Bk.
IV,
Alhazen
argues
that
reflection of a
sensible
ray
must occur
from
a sensible
point, having
a
latitude
equal
to
that
of
the
ray;
although
he
gives
no
indication,
Al-
hazen
might
have considered
the same
analy-
sis applicable in the present case. Cf. ibid., IV,
Sec.
16,
p.
112.
37
Note
that
all
tunics
(or,
more
accurately,
all those before
the
back
surface
of
the lens-
see n.
39)
are
concentric
with
the
cornea,
so
that
rays perpendicular
to the
cornea are
per-
pendicular
to all tunics.
328
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ALHAZEN'S
THEORY OF
VISION
linear,
they
maintain a
fixed
order,
and "the
form will be
arranged
on
the
surface
of the
glacial
humor
just
as it is on
the
surface of the visible ob-
ject."38
A
one-to-one
correspondence,
which
insures clear
and
unconfused
perception, has thus been established between points on the object and points
on
the
surface of
the
glacial
humor.39
When the
propagation
of
forms
in
straight
lines
is
expressed
in
geometrical
terms,
one
has
a
pyramid
with
base on
the
visible
object
and vertex
at the
center
of
the
observer's
eye.40
From
each
point
on the
base,
a line
can be
drawn to
the
vertex
in the
eye,
representing
the
path
of
the form
by
which
vision
of that
point
is achieved.
This
pyramid
is a
geometrical representation
of
the
process
of
sight
and aids the
investigator
in
understanding
the
process.41
Alhazen
has
thus
succeeded
in
restoring
the
visual
pyramid
of
Euclidean
and
Ptolemaic
optics
and,
thereby,
the inherent
intelligibility
of
Greek
emission
theories of vision. But,
significantly,
he has done so within an intromis-
sion
framework.
For
the
first
time
an
intromission
theory
of vision
has
be-
come
a
viable
alternative,
adequate
to
compete
on
geometrical
as
well
as
physical
and
physiological
terms
with
the
theory
of visual
rays.
Alhazen's
theory
of
vision,
as
presented
above,
is confined
mostly
to the
first
of
the
seven
books
of
his
Perspectiva.
Book
II contains
his
psychology
of
per-
ception.
Book
III continues
in a
psychological
vein,
dealing
with
the errors of
vision,
including
those
associated
with
binocular
vision. Books IV
and V
are
concerned
with
reflection from
plane
mirrors and curved
mirrors
(both
con-
cave
and
convex)
of
spherical,
conical,
and
cylindrical figure.
In Book VI
Al-
hazen
discusses
the errors
in
perception
resulting
from
vision
by
reflected
rays
(i.e.,
errors
in
number,
location,
and size
of
images).
Book VII
is
devoted
to
refraction
of
rays.
There
is
no doubt
that
Alhazen
contributed
to
geometrical
studies
of
reflection
and
refraction,
but
his
significant
innovations
were
limited
to
his
theory
of
vision.
Although
he
extended
Ptolemy's geometrical
optics
to
new cases
and to
a
higher
level of
sophistication,
it
was
still Ptole-
38
Ibid.,
I,
Sec.
24,
p.
15.
39
In
this
paper
I
have
not
probed
deeply
into Alhazen's
psychology
of
perception.
In
brief, he argues that vision is not completed in
the
glacial
humor
(lens),
but
by
the
virtus dis-
tinctiva
belonging
to the
ultimum
sentiens,
which
is situated
in
the anterior
part
of
the
brain.
The forms
of
light
and
color
penetrate
the
glacial
humor
and,
at
the
interface
separat-
ing
the
glacial
humor and
vitreous humor
(which
is
before
the center
of
the
eye),
are
re-
fracted
away
from
the center
of the
eye
and
never
actually
converge
to a
vertex.
The forms
are
then
conducted
through
the
vitreous
humor
and
hollow
optic
nerve-all the
time maintain-
ing
their
proper
disposition-to
the
optic
chias-
ma, where they join the forms from the other
eye.
They
continue
through
the visual
spirit
to the anterior
part
of the brain.
Cf.
Opt.
thes.,
I,
Ch.
5,
pp.
15
ff.;
II,
Ch.
1,
pp.
24
ff.
For
a
full-length
study
of
Alhazen's
psychology
of
perception,
see Hans
Bauer,
Die
Psychologie
Alhazens
(Beitrdge
zur Geschichte der Philoso-
phie
des
Mittelalters, 1911, 10,
Pt.
5).
The
same
general
scheme
is found
in
the
writings
of
Bacon,
Pecham,
and
Witelo;
on
differences
between the psychology of Alhazen and his
Latin
followers,
see
Vescovini, Studi,
Chs.
4,
7.
40
In
works translated
from
the
Arabic,
the
term
pyramis
is used even
when the
figure
has
a round base and
hence
could
aptly
be
desig-
nated
by
the term
conus. See
Marshall
Clagett,
"The De curvis
superficiebus
Archimenidis: A
Medieval
Commentary
of
Johannes
de Tinemue
on Book
I
of the
De
sphaera
et
cylindro
of
Archimedes,"
Osiris,
1954,
11:298.
41
Opt.
thes., I,
Sec.
24, p.
15.
An earlier
point
can
now
be clarified: Alhazen
is
willing
for mathematicians to talk about imaginary rays
issuing
from
the
eye
in
pyramidal
form
because
if
rays
emanate
from the
center
of the
eye,
the
pyramid
of vision is formed
without further
ado;
there
are
no
rays
not
perpendicular
to
the surface
of the
eye
to
interfere with
the
geometrical
scheme.
329
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DAVID C.
LINDBERG
maic
optics
that
he
was
extending.
He
played
the
game
with
more
finesse than
Ptolemy,
but
it
was still
the
same
game.
The
novelty
of Alhazen's
theory
of
vision had no
influence on
traditional
geometrical
optics;
not
only
can
geo-
metrical optics be pursued without commitment to any particular theory of
vision,
but
Alhazen
was
even
willing
to
allow
"mathematicians"
to
continue
to
express
themselves
in
terms of the
discredited emission
theory.
II
The
wave of
translations from
Arabic to
Latin in
the
twelfth and thirteenth
centuries
included Alhazen's Kitab
al-manazir. The name
of
the
translator
and
provenance
of
the
translation
are
unknown,
but the treatise
was
evidently
translated in
the
late twelfth or
early
thirteenth
century.
The earliest
known
Western citation
appears
in a work
of
Jordanus
de
Nemore,42
who flourished
in
the
early
part
of the
thirteenth
century,
but
its
diffusion
in
the first half
of
the thirteenth
century
was not
sufficiently
wide to
have
brought
it to the
attention
of
Robert
Grosseteste,
who wrote
on
optics
in the
first third
of the
century.43
The
full
impact
of Alhazen's
new
optical
theories is first seen
in
the
writings
of
Roger
Bacon,
John
Pecham,
and
Witelo,
all
of whom wrote
on
optics
in
the
1260's
and
1270's.44
The
eagerness
with
which
Alhazen's
optics
was
received
in the
West
was
doubtless
due to its
promise
of
contributing
to
an
already flourishing
optical
tradition. Kept alive (if barely) by the encyclopedic tradition of the early
Middle
Ages, optical
theory
was nourished
dramatically
by early
translations
of scientific
works
into
Latin and
brought
into
prominence
by
Robert Grosse-
teste,
who
revived
optical
and other scientific studies
at Oxford
early
in
the
thirteenth
century.45 Among
the
optical
treatises available to
Bacon, Pecham,
42
See
Marshall
Clagett,
Archimedes
in the
Middle
Ages,
Vol.
I
(Madison:
Univ. Wisconsin
Press,
1964), p.
669.
43
Richard
C.
Dales,
"Robert Grosseteste's
Scientific
Works,"
Isis, 1961,
52:394-402,
dates
Grosseteste's optical works between 1231 and
1235.
Occasionally
al-Bitriji (d. early
13th cen-
tury)
is
quoted
as
having
asserted
that Alhazen's
Perspectiva
was
circulating
in the West
during
his
lifetime:
"Nam
licet
perspectiva
Alhacen
sit
in
usu
aliquorum
sapientium
Latino-
rum.
.
... (Cf.
Lucien
Leclerc,
Histoire
de la
medecine
Arabe,
Vol.
II
[Paris:
E.
Leroux,
1876],
p.
516.)
However,
this is
not a
quotation
from
al-Bitrfji,
but
from a
fragment
of the
Opus
tertium of
Roger
Bacon,
which
(in
Paris,
Bibliotheque
Nationale,
Latin MS
10264,
fol.
186)
is
wrongly
entitled
Liber tertius
Alpetra-
gii; cf. Un fragment inedit de l'Opus tertium
de
Roger
Bacon,
ed. Pierre Duhem
(Quaracchi:
Collegium
S.
Bonaventurae,
1909),
p.
75.
44
On medieval
Western
optics
in
general,
see
A. C.
Crombie,
Robert Grosseteste and
the
Ori-
gins
of
Experimental
Science,
1100-1700
(Ox-
ford: Clarendon
Press,
1953),
and
Vescovini,
Studi.
On
Pecham,
see
my
"The
Perspectiva
communis of
John
Pecham."
All
texts
and
translations
from Pecham's
Perspectiva
com-
munis are
drawn
from
my forthcoming
edition
(Univ.
Wisconsin
Press).
The
best sources
on
Bacon's
optics
are in
Roger
Bacon
Essays,
ed.
A. G. Little
(Oxford:
Clarendon
Press,
1914),
but further work
is
in
order.
However,
Bacon's
theories are
readily
accessible
in
the
English
translation
of
his
Opus
majus
(The
Opus
Ma-
jus
of
Roger
Bacon,
trans. R. B.
Burke,
Phila-
delphia:
Univ.
Pennsylvania
Press,
1928).
On
Witelo,
see
Clemens
Baeumker,
Witelo,
ein
Phi-
losoph
und
Naturforscher
der XIII.
Jahrhun-
derts
(Beitrige
zur
Geschichte
der
Philosophie
des
Mittelalters, 1908,
3,
Pt.
2);
Baeumker
bases
some
of his
argument
on the
conclusion
that
Witelo was
the
author
of
De
intelligentiis,
a
conclusion now clearly recognized as false. On
Witelo,
see
also
my
introduction
to
a forthcom-
ing
facsimile
reprint
of
the Risner edition
(New
York:
Johnson
Reprint,
Sources of
Science).
45
On Grosseteste's
sources,
see
Crombie,
Grosseteste,
pp.
116-117.
330
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ALHAZEN'S THEORY OF VISION
and
Witelo were
works
by
Greek,
Muslim,
and Latin authors.
There
were,
of
course,
Alhazen
and
Grosseteste.
In addition to
these
two,
Pecham
cites
al-Kindi, Aristotle,
the
pseudo-Euclidean
Catoptrica,
and a medieval
abridge-
ment of Euclid's Optica; he appears to have known, also, Alhazen's De specu-
lis comburentibus
(a
short work
distinct
from the
Perspectiva),
and he
may
have used
Ptolemy's
Optica.46
Bacon cites
all
these on
optics,
as
well as
Avi-
cenna,
Averroes, Tideus,
Constantinus
Africanus,
and
Augustine.
Though
but one of
many
authorities,
Alhazen
exerted
by
far
the
dominant
influence. Bacon
continually
cites him
by
name,
and
Pecham
and Witelo
consciously
patterned
their
major optical
works
after his
Perspectiva,
respec-
tively
condensing
and
expanding
its treatment.
Pecham
continually
bows
to
the
authority
of
Alhazen,
whom he
cites
as
"the Author"
or "the
Physicist";
and
Risner,
publisher
of
the
printed
works
of Alhazen
(Latin
text)
and Witelo
in a
single
volume,
has indicated their close
relationship
by
elaborate cross-
references. But
aside from
citations
and
format,
the theories of vision
ex-
pressed
by
Bacon,
Pecham,
and
Witelo
are
essentially
the
same as
Alhazen's.47
All
describe
the
anatomy
of the
eye similarly,
with
only
small
differences
in
detail.
Vision,
according
to
Bacon,
Pecham,
and
Witelo,
occurs
through rays
issuing
from
the
visible
object
and
falling
perpendicularly
on
the
surface
of
the
eye
and
the
glacial
humor.
Nonperpendicular
rays
are
refracted
and con-
tribute to vision
only
incidentally. Through
the
visual
pyramid,
consisting
solely
of
perpendicular
rays issuing
from the
object
and
converging
toward
a
vertex
at the center
of
the
eye,48
the
forms of the
light
and color of the
ob-
ject
are
arranged
on
the
surface
of
the
glacial
humor
precisely
as
on
the
sur-
face
of
the
object; consequently
a
one-to-one
correspondence
is
established,
which insures
clarity
of vision. In
order to
establish this
theory
of
vision,
Bacon, Pecham,
and
Witelo even
rely
on the
same
evidence and
the same
arguments
as
Alhazen. All
four,
for
example,
cite the
pain
experienced
by
the
eye
in
looking
at
bright lights
as
evidence that
light
and
color make some
kind
of
impression
on
the
eye,
and
Pecham
(like Alhazen)
begins
his
treatise with
a
description
of such
evidence.49
It
should
be
evident,
then-and historians
have
long agreed
on this-that
the main outlines of Alhazen's theory of sight, as well as his more abstract
geometry
of
image
formation
by
reflection and
refraction
and
many
minor
details,
were
incorporated
in
the
optical
works of
Bacon,
Pecham,
and Witelo.
Alhazen's
theory
was
comprehensive
and
systematic;
it
was
superior
in
almost
every respect
to
anything
the West
had
known
before.
To
some extent it also
46
For a
fuller
discussion of Pecham's
sources,
away
from the
center of the
eye
at
the
interface
see
my
edition
of
the
Perspectiva
communis.
between the
glacial
humor and
vitreous humor.
47
Crombie,
Grosseteste,
gives
Bacon,
Pecham,
This
prevents
inversion
or reversal of the
forms
and Witelo far
too much
credit
for
experi-
as
they
are
conducted
through
the
optic
nerves;
mental
or observational
prowess
and attaches
cf.
n.
39.
too
much
significance
to Grosseteste's influence
49
Perspectiva
communis, I,
Prop.
1. On
Al-
on their
optical
theories. For the most
part,
hazen, see above.
Cf.
Bacon, The
Opus
Majus
Bacon, Pecham,
and Witelo did not
observe,
of
Roger
Bacon,
ed.
J.
H.
Bridges (London:
experiment,
or read
Grosseteste;
they
read Al-
Williams and
Norgate,
1900),
V,
1,
Dist.
5,
Ch.
1,
hazen
and
other Islamic
and Greek
authorities.
Vol.
II,
pp.
30-32;Witelo,
Opticae
libri
decem,
48
As in
Alhazen's
theory,
the
rays
do not
ed.
Risner
(bound
with
Opt.
thes.),
III,
pp.
actually converge
to
a
vertex,
but are
refracted
87-88,
91.
331
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DAVID C.
LINDBERG
filled
an
intellectual
void;
the works of
Aristotle, Euclid,
Ptolemy,
al-Kindi,
and
Grosseteste
said little about the anatomical
and
physiological
details
of
sight.
These
treatises dealt
primarily
with
geometrical optics,
on
which
they
were in substantial agreement with Alhazen. Conflict was therefore at a mini-
mum.
Where earlier
authorities had
spoken,
Alhazen
agreed;
where
they
had
been
silent,
he
provided
a
theory
that was
overwhelmingly
systematic
and
comprehensive.
Nevertheless,
strong
reservations have been
expressed
about the assimila-
tion of
Alhazen's
optics
in
the
West,
particularly
with
regard
to
the
properties
of
the
emanated
entity.
Vasco
Ronchi,
one of the
most articulate and
prolific
writers
on
the
history
of
optics
in
recent
years,
has
repeatedly
characterized
medieval Western
optics
as a
corruption
of Muslim
views and a return
to the
less
satisfactory
theories
of
Greek
antiquity.
According
to
Ronchi,
Alhazen
had set the
theory
of
sight
on the road
leading
to
Kepler
and modern
optics
by
decomposing
"l'objet
visible en
elements
punctiformes,
ce
qui
faisait
perdre
a
la
vision
de
l'objet
lui-meme le caractere d'une
operation
globale
qu'on
ne
pouvait
scinder."
50
By treating
the
object
a
point
at a
time,
Alhazen
had
escaped
the
absurdities of
Epicurean
eidola,
while
still
operating
within
an intromission
framework.
"Les
'eidola' et les
'ecorces,'"
Ronchi
writes,
"sont mortes."
51
And
so
they
were. The
question
is whether
they
stayed
dead.
Ronchi thinks
not.
Eidola and
rinds
reappeared,
he
insists,
in
the
"species"
of
Western
optics.
Scholastic
scientists,
with a few
exceptions, attempted
to
reconcile eidola
(poorly disguised
in
new
terminology)
with
Alhazen's revo-
lutionary
contributions. The
result,
Ronchi
insists,
was
"une
construction
grotesque."52
Simply put,
Ronchi
feels that
species
are eidola.
He sums
up
this
viewpoint
most
forcefully
when,
after
describing
the
diffusion
of
Al-
hazen's ideas and
the
inability
of Western
scientists to
understand
them,
he
writes,
What ensued was
an
indescribable
atrophy
of
thought.
Ideas tended
more
or
less
to cluster about
the
doctrine
of
species,
a
new edition of
the ancient
eidola. These
species,
however,
were
produced
by
the
lumen,
when
it im-
pinged upon
a
body,
and
they
moved
along
the observer's visual
rays
as
though along rails guiding them toward the eyes. During this motion they
contracted
in
order
to
be able
to
enter the
pupil.
The contraction no
longer
constituted
a serious
obstacle
[as
it had
in
ancient
times]
because
Ibn al-
Haitham's
mechanism had
provided
a sort
of
justification
for it.
In
other
words,
an effort
was made
to
combine the
classical
with
the
new.
The
merger
was a
monstrosity,
with which the
philosophers
and
mathema-
ticians
of
the later Middle
Ages
tried
to
reason when confronted
by
optical
problems.53
An
important question
remains: To
precisely
which
Western
thinkers
is
Ronchi's
analysis
to be
applied? According
to
the
most
sympathetic
interpre-
tation, Ronchi recognizes only three Western philosophers who assimilated
Alhazen's
revolutionary
theories
regarding
the
process
of
sight:
Bacon,
Pe-
50
Ronchi, Histoire,
p.
38.
53
Ronchi,
Optics,
The
Science
of
Vision,
51
Ibid.,
p.
37.
On
eidola,
see below.
trans.
Edward
Rosen
(New
York: New
York
52
Ibid.,
p.
55.
Univ.
Press,
1957), p.
32.
332
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ALHAZEN'S
THEORY OF
VISION
cham,
and Witelo. On the
other
hand,
he
singles
out Dante
Alighieri
(d.
1321),
Gregorius
Reisch
(d.
1525),
and
Giambattista
della Porta
(d.
1615)
as
examples
of men
who
failed to
grasp
Alhazen's
theory,
and
from
an examina-
tion of their works purports to demonstrate the universal (or almost uni-
versal)
failure
of scientists of
the
late Middle
Ages
to
understand
or
adopt
Alhazen's
important
innovations.54
In
Ronchi's
view,
then,
Alhazen's ideas
were assimilated
by
Bacon, Pecham,
and Witelo
in the thirteenth
century,
but
afterwards
forgotten,
rejected,
or misunderstood
until the time of Mau-
rolycus
and
Kepler
at
the end
of
the
sixteenth
century.
But
Ronchi has
often been
misleading,
and this
"sympathetic
interpreta-
tion" of
Ronchi's views
does not
jump
out at
the
reader of his works.
In
his
main
work
on
the
history
of
optics
Ronchi
gives
not the
slightest
indication
that Bacon and Pecham
had
anything
to do
with the
diffusion of Alhazen's
ideas.55 Moreover, Ronchi's condemnation of the idea of
species
is
sweeping;
in no
work does he admit
any
exceptions
to the
grotesqueness
or
acknowledge
any
varieties of the idea.
Thus
he
leaves the
clear
impression
that
the
species
concept
in all its
forms was a
departure
from the
teaching
of
Alhazen;
and
this
impression
is
reinforced
by
the
fact that
in
most of
Ronchi's works
only
Witelo,
who
never
used the
term
"species,"
is
recognized
as a
faithful follower
of
Alhazen.56
Finally,
in
his
most
explicit
statement of the
period
during
which
this
"atrophy
of
thought"
occurred,
Ronchi
writes,
...
pendant
quatre
ou
cinq
siecles
les
idees
d'Alhazen
n'ont
eu aucune
consequence appreciable. Si, pendant ce laps de temps, on recherche dans
les
oeuvres le
plus
remarquables
du monde
occidental
quelles
etaient
les idees
predominantes
au
sujet
de la
lumiere,
on
retrouve celles de la
period
grecque.57
Since
the
light
of
understanding
dawns
again
toward
the end
of
the
sixteenth
century
with
the
printing
of Alhazen's
Perspectiva,58
four or
five centuries
take one back
to the
eleventh
or twelfth
century-before
the
translation of
Alhazen's
work
into Latin.
Consequently,
the
passage
must
be
interpreted
as
asserting
either
that
Alhazen's
theory
was
not
adopted
by
Bacon, Pecham,
54
Histoire,
pp.
45-49,
57-73.
These
three
men are hardly the best representatives (with
the
possible
exception
of della
Porta)
of medie-
val
or even late-medieval Western
optics.
Against
Ronchi one
might
even
argue
that
since
Dante, Reisch,
and
della
Porta failed
to
grasp
Alhazen's
theories,
they
could
not
have
been serious students
of
optics.
At
any
rate,
Ronchi's thesis would be more
impressive
if
it
could
be
demonstrated
with
respect
to
such
competent
natural
philosophers
and
optical
theorists as Theodoric of
Freiberg,
Blasius of
Parma,
and Friedrich
Risner.
55
Ronchi's main work on the
history
of
optics
is Storia della
luce,
translated
into
French
as
Histoire
de
la
lumiere.
The
only
work
(to
my knowledge)
in which
Ronchi
mentions
Bacon and
Pecham as followers of
Alhazen
is
his
Optics,
The
Science
of
Vision,
p.
31,
and
there
they
receive but two
sentences.
56
Histoire,
pp.
44-45;
Ronchi,
"Sul con-
tributo di Ibn-al-Haitham alle teorie della vi-
sione e
della
luce,"
in
Actes du VIIIe
Congres
International d'Histoire
des Sciences
(Jerusalem,
1953),
p.
520.
57
Histoire,
p.
45.
58
Ronchi
(ibid.)
asserts that
Risner,
publisher
of the
1572
edition,
was
also its
translator
from
Arabic
to
Latin.
Elsewhere
he
identifies
Witelo
as
the
translator
("Complexities,
Advances,
and
Misconceptions
in the
Development
of the
Science
of
Vision: What is
being
Discovered?"
in
Scientific
Change,
ed. A.
C.
Crombie [New
York:
Basic
Books, 1963],
p. 546).
Actually,
neither is
correct. The
book was rendered
into
Latin
by
an unknown
translator late
in
the
12th
or
early
in the
13th
century,
more
likely
the
latter.
Risner
merely
edited
the medieval
trans-
lation,
as he
relates
in
the
preface
to
his
edi-
tion;
Witelo
had
nothing
whatsoever to
do
with
it.
333
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DAVID C. LINDBERG
and
Witelo or
that
its
incorporation
in
their works was not an
"appreciable
consequence."59
But,
in
fact,
Bacon,
Pecham,
and
Witelo
fully
assimilated
the
optics
of
Alhazen, and their works had a very appreciable influence on succeeding
generations.
The
unusually
large
number
of
manuscripts
and
printed
editions
of
Pecham's
Perspectiva
communis
(as
least
47
extant
manuscripts
and
10
printings)
and
Witelo's
Perspectiva (numerous
manuscripts
and 3
printings)
testifies to
the diffusion
of Alhazen's ideas
throughout
the later Middle
Ages.
Nor were these
a matter
of
reproduction
without
understanding;
for
example,
the
1542
edition
of Pecham's
Perspectiva
communis was
altered
extensively
by
its
editor,
the
mathematician and instrument-maker
George
Hartmann
(1489-1564),
with
no
distortion of the essential ideas.
Moreover,
Pecham's
Perspectiva
communis
was the
subject
of a
number
of
commentaries,
the
authors of which did not fail to understand the contents.60 Eventually Pe-
cham's book became the standard
optical
text
in the
medieval
university,
so
that even
an
elementary
education
in
optics
provided
an
understanding
of
Alhazen's
theories. If late
medieval
writers on
optics
did not
preserve
Al-
hazen's ideas on vision in
their
works,
it
was not because
they
failed
to
comprehend,
but because
they
were
asking
different
questions.6'
Nevertheless,
Ronchi's
work has
raised
the
question
of
the
relationship
of
medieval
species
to
Alhazen's
forms and
Epicurean
eidola;
and
if
I
disagree
with
Ronchi's
answer,
I
at least endorse his
question,
for
it
is most
important,
deserving
a detailed
and
unambiguous
answer.
I
will devote the
remainder
of this essay to it, restricting myself for the present to the concept of species
in
its
thirteenth-century
form;
62
it will
be
my
thesis
that from an
optical
standpoint
the Western
idea
of
species
is
indistinguishable
(with
one
impor-
tant
exception)
from Alhazen's
concept
of form.
Since
the
standard of
com-
parison,
as Ronchi has
posed
the
problem,
is the
Epicurean
eidolon,
let
us
first
consider
the
latter.
Epicurus
describes eidola
in his Letter to Herodotus:
"particles
are
con-
tinually
streaming
off
from the surface
of bodies.
. . . And those
given
off,
for
a
long
time
retain
the
position
and
arrangement
which their atoms
had
when
they
formed
part
of the solid bodies."63
Sight
occurs as these
eidola
enter
the
observer's
eye:
We
must
also consider that
it
is
by
the entrance of
something
coming
from
external
objects
that we
see their
shapes
and
think of
them. For external
59
Ronchi
has
already
admitted,
on the same
larizers
or amateurs like Dante and Reisch.
For
page,
that
Witelo
faithfully
followed
Alhazen.
example,
Theodoric
of
Freiberg,
in his
De
luce,
However,
more
recently
Ronchi has
written
was concerned not
with
geometrical
optics
but
that
"at
the
end
of
the
thirteenth
century
with
a causal account
of
light;
cf.
William
A.
A.D
. .
.
[the
work
of
Ibn
al-Haitham]
had as
Wallace,
O.P.,
The
Scientific
Methodology
of
yet
had
no
impact
on the
Western
world" Theodoric
of
Freiberg (Fribourg,
Switzerland:
(Scientific
Change, p. 545).
Univ.
Press,
1959), pp.
152-161.
60For a discussion of the medieval commen-
6
I plan to deal with the later development
taries
on the
Perspectiva
communis,
see
my
A
*
v
aries
onhe
Perspectiva
communis,
see
my
of
the
concept
of
species in
a
future
paper.
edition,
n.
44 above.
The
influence
of Al-
hazen's
Perspectiva
is
explored
in more detail
63
Diogenes
Laertius,
Lives
of
Eminent
Phi-
in
my
introduction
to the
forthcoming
reprint
losophers,
X,
48,
trans. R. D. Hicks
(London:
of the
Risner
edition. William
Heinemann,
1925),
Vol.
II,
pp.
577-
61
I
have
in
mind
serious
scientists,
not
popu-
579.
334
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ALHAZEN'S THEORY OF
VISION
things
would
not
stamp
on us
their own nature
of
colour
and
form
through
the
medium of
the
air
which is
between them and
us,
or
by
means
of
rays
of
light
or
currents of
any
sort
going
from
us
to
them,
so
well as
by
the entrance
into our
eyes
or
minds, to whichever their size is suitable, of certain films
coming
from
the
things
themselves,
these films
or
outlines
being
of the same
colour and
shape
as the external
things
themselves.64
Thus eidola
(material images
or
skins)
are
stripped
from the outer surfaces
of
objects
and
propagated
through
space
as
coherent
units.
Because
they
are
of the
same
shape
and color
as
the
object,
they
faithfully
communicate
the
latter
to the
observer. But
the
fatal
objection
to
the
Epicurean
theory,
be-
sides
its
generally
simplistic
character,
is the
problem
of
shrinking
the
eidola
of
large
objects,
so
as
to
squeeze
them
into the observer's
eye,
without
altering
them in
shape
or
color-and this for all
possible
distances between
the
object
and the
observer.
Now
it
is
clear that
Alhazen's forms
have
little in common
with
Epicurean
eidola. In
the first
place,
Alhazen is
willing
to consider the form
of each
point
(or
small
part)
of the
object independently:
he
refers,
for
example,
to
"the
form of the
light
and
color
that
comes
from
any
point
of
the
visible
object
to the surface
of
the
eye."
65
Since
he is
dealing
with
an
image
of
each
point
rather than a
single
replica
of the
whole
object,
squeezing
images through
the
pupil
is no
problem. Secondly,
Alhazen's
forms
are
not
images
in the
Epicurean
sense.
They
do
not
consist of
pieces
of the
object;
they
are not
replicas. Rather they are powers representative of the object, capable of
producing
effects
in a
recipient.
However,
noting
Alhazen's
explanation
of
reflection on
the
analogy
of mechanical
rebound,
Ronchi
writes,
"The
idea
that the
rays
of lumen are
the
trajectories
of minute material
corpuscles
is
already
expressed
in
his
work."
66
But
this
is to misunderstand
Alhazen. The
analogy
of
mechanical rebound is meant
to elucidate the
equal angles
of
reflection,
not
the nature of the
reflected
entity.
Schramm,
basing
his
argu-
ment
on a
study
of both
the
Latin and
Arabic
texts
of the
Kitab
al-manazir,
has concluded
that
Alhazen's
form is not
a
three-dimensional
body,
but rather
that which is
impressed
on a
body through qualitative change.67
Form for
Alhazen is thus
very
close
to
Aristotelian
form.
The
concept
of
species,
as
applied
to
optics,
had
its
origin
in
the
Neopla-
tonic
doctrine
of emanation. Plotinus
(d.
270
A.D.)
maintained
that causation
64
Ibid., X,
49,
Vol.
II,
p.
579.
For a
good
p.
112.
Alhazen also elucidates
refraction
by
account
of the
Epicurean theory
of
vision,
see
means of mechanical
analogies
(ibid.,
VII,
Sec.
Cyril Bailey,
The
Greek
Atomists
and
Epicurus
8,
p. 241).
(Oxford:
Clarendon
Press,
1928),
pp.
406-413;
67
Schramm,
Weg
zur
Physik, p.
216.
A.
I.
cf.
Haas, "Lichttheorien,"
pp.
362-370.
Sabra
writes,
".
.
he
[Alhazen]
denies
that
65
Opt.
thes.,
I,
Sec.
18,
p.
9.
Alhazen also
light
is a
body
.
.
."
("Explanation
of
Optical
speaks
of
a
single
form for
the
whole
object
Reflection
and
Refraction:
Ibn-al-Haytham,
on occasion. Descartes, Newton," Actes du dixieme congres,
66
Optics,
Science
of
Vision,
p.
30.
Cf.
Ron-
Vol.
I,
p. 551).
Cf.
J.
Baarman,
"Abhandlung
chi,
Histoire,
p.
42,
and
Ronchi,
"The Evolu- iiber
das
Licht
von
Ibn
al-Haitham,"
Zeit-
tion
of the
Meaning
of
'Light'
in
Natural
Phi-
schrift
der Deutschen
Morgenldndischen
Ge-
losophy,"
in Actes
du
dixieme
congres
interna-
sellschaft,
1882, 36:197-199;
this is the
Arabic
tional d'histoire
des
sciences
(Ithaca, 1962),
Vol.
text and German translation
of
Alhazen's
short
II,
p.
725.
Cf.
Alhazen,
Opt.
thes., IV,
Sec.
18,
treatise
On
Light.
335
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DAVID
C.
LINDBERG
may
occur
through
a
process
of emanation.
Indeed,
all
things
tend to emanate
their
power
outside
themselves.
In
The Enneads
Plotinus
writes,
All
existences,
as
long
as
they
retain their
character, produce-about
them-
selves,
from their
essence,
in
virtue
of
the
power
which
must be
in
them-some
necessary,
outward-facing
hypostasis
continuously
attached
to
them and
rep-
resenting
in
image
the
engendering archetypes:
thus
fire
gives
out
its
heat;
snow is
cold
not
merely
to
itself;
fragrant
substances
are
a notable
instance;
for,
as
long
as
they
last,
something
is diffused
from them
and
perceived
where-
ever
they
are
present.68
Light
emanating
from
a
luminous
body
is another
example
of
the same
effect.
In
the
same
vein,
Avicebron
(d.
c.
1058),
whose Fons
vitae was available to
Grosseteste
and other
thirteenth-century
writers,
argues
that
powers
and
rays
emanate from all simple substances on the analogy of the emanation of light
from
the sun: "The
essences of
simple
substances
do
not issue
forth;
it is
rather their
powers
and
rays
that
flow forth
and
spread
abroad. . . .
Just
as
light
flows
from
the sun into
the
air,
. . .
so
every
simple
substance extends
its
ray
and its
light
and
diffuses them into
that which
is
inferior.
".
.
69
These
ideas were
developed
by
Grosseteste into
the
doctrine of the multi-
plication
of
species.
As he
expresses
this
doctrine
in
his
De
lineis,
angulis
et
figuris,
A natural
agent
propagates
its
power
from
itself
to
the
recipient,
whether it
acts on the senses or on matter. This power is sometimes called species, some-
times a
similitude,
and is
the
same whatever it
may
be
called;
and it
will send
the same
power
into
the
senses
and into
matter,
or
into its
contrary,
as
heat
sends the same
thing
into
the
sense of
touch
and
into
a cold
body.
For
it
does
not act
by
deliberation
and
choice,
and
therefore it
acts
in
one
way,
whatever
it
may
meet,
whether
something
with sense
perception
or
some-
thing
without
it,
whether
something
animate or
inanimate. But the
effects
are
diversified
according
to
the
diversity
of
the
recipient.70
Every
natural
agent
propagates
its
power
from
itself
to
surrounding
bodies,
and this
power
is called
"species"
because the
effects bear
a
resemblance to
the
agent.
Hot bodies, for
example,
emanate
species
that
produce
heat in the
recipient,
and
bright
bodies
emanate
species
that
produce brightness
in
the
observer's
eye.71
Roger
Bacon
and
John
Pecham
adopted
Grosseteste's doctrine
of
the multi-
68
Plotinus,
The
Enneads, V, 1, 6,
trans.
71
As
Grosseteste
points
out
in
the
passage
Stephen
MacKenna
(2nd
ed.,
London:
Faber
quoted,
the effects do
not
always
resemble the
and
Faber,
1956),
p.
374.
agent;
but this is
an
insight
not
possessed
by
69
I
have
translated
this from the Latin text
all
writers on the
subject,
and
even
in
Grosse-
in Avencebrolis
(Ibn Gebirol),
Fons
vitae, III,
teste the
effects
are never
totally
unlike the
52,
ed. Clemens
Baeumker
(Beitrdge
zur Ge-
agent.
Bacon insists that
"haec
species
sit similis
schichte der Philosophie des Mittelalters, 1895, agenti" (De multiplicatio specierum, ed. J. H.
1), p.
196. Similar ideas
on the
multiplication
Bridges,
bound with the
Opus
majus,
Vol.
II,
of
power
had
been
expressed by
al-Kindi,
and
p.
410).
Cf.
Avicebron,
Fons
vitae,
III, 53;
these
may
have been
known to
Grosseteste as
Charles K.
McKeon,
A
Study
of
the Summa
well;
cf.
Vescovini, Studi,
Ch.
3.
philosophiae
of
the
Pseudo-Grosseteste
(New
70
Quoted
by
Crombie, Grosseteste,
p.
110.
York: Columbia Univ.
Press,
1948),
pp.
87,
94.
336
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ALHAZEN'S THEORY
OF
VISION
plication
of
species
and
incorporated
it
in
their theories
of vision drawn from
Alhazen.
Bacon,
the more
vocal
on the
subject
of
species, explains,
For
every
efficient
cause
acts
through
its own
power,
which
it
exercises
on
the
adjacent
matter,
as
the
light
[lux]
of
the
sun
exercises
its
power
on the
air
(which
power
is
light
[lumen]
diffused
through
the
whole world from
the
solar
light
[lux]).
And
this
power
is called
"likeness,"
"image,"
and
"species"
and is
designated
by many
other
names,
and it is
produced
by
substance
as
well
as
accident.
. .
.
This
species
produces
every
action
in
this
world,
for
it
acts
on
sense,
on the
intellect,
and
on
all
the matter
of
the world
for
the
generation
of
things.72
Pecham's view is similar:
Every
natural
body,
visible or
invisible,
diffuses
its
power
radiantly
into
other
bodies. The proof of this is by a natural cause, for a natural body acts outside
itself
through
the
multiplication
of its
form.
Therefore the nobler
it
is,
the
more
strongly
it acts. And
since
action
in
a
straight
line
is easier
and
stronger
for
nature,
every
natural
body,
whether visible
or
not,
must
multiply
its
species
in
a
continuous
straight
line;
and this is
to
radiate.73
Now
the central
question
of this
essay
is
whether or
not
this
concept
of
species
was
"a
new
edition
of
the
ancient
eidola." The answer
is
clear-no
As
these
quotations
from
Bacon and Pecham
indicate,
species
are not material
replicas,
films
of
matter
peeled
from
the outer surface
of
the
object
and
pro-
pelled through space, but powers or forms diffused from one point to another
through
the matter
already
there.
The actual mode of
multiplication
or
propagation
of
species
is
very
clearly
described
by
Bacon:
But a
species
is
not
body,
nor is it
moved as a whole
from one
place
to an-
other;
but that which
is
produced
[by
an
object]
in the first
part
of
the
air
is not
separated
from that
part,
since
form cannot
be
separated
from the
matter
in which
it is
unless
it
should
be
soul; rather,
it
produces
a likeness
to itself
in the
second
part
of the
air,
and
so
on.
Therefore
there
is no
change
of
place,
but a
generation multiplied
through
the different
parts
of
the me-
dium;
nor
is
it
body
which
is
generated
there,
but
a
corporeal
form;
. . . and
it is not produced by a flow from the luminous body, but by a drawing forth
out
of the
potentiality
of
the
matter
of
the
air.74
The
multiplication
of
species
is more like the
propagation
of
waves
than
like the
motion
of
projectiles.
72
Opus
majus,
IV,
Dist.
2,
Ch.
1,
Bridges,
Huius
probatio
est
per
causam
naturalem,
Vol.
I,
p. 111;
cf.
De mult.
spec., Bridges,
Vol.
quoniam
corpus
naturale
agit per
formam suam
II,
pp.
407-418.
For a careful discussion
of se extra se
multiplicantem.
Ergo
quanto
nobi-
Bacon's
concept
of
species,
see
Vescovini, Studi,
lior
tanto est fortior
in
agendo.
Et
quia
actio
pp.
57-60. Bacon
distinguishes
between
lux
in
directum
est facilior
et fortior
nature,
necesse
and
lumen
in the
passage quoted,
but admits
est ut omne
corpus
naturale seu
visibile
seu
in De mult. spec., "Sed tamen usualiter lucem non visibile suam speciem multiplicet in con-
accipimus
pro
lumine
et e
contrario"
(Bridges,
tinuum et
directum,
et hoc
est
radiare"
(I,
Vol.
II,
p.
409).
Prop.
27
of Pecham's revised version
of Pers.
corn.).
Cf.
II,
Prop.
5.
73
"Omne
corpus
naturale visibile seu non
74
Opus
majus,
V,
1,
Dist.
9,
Ch.
4,
Bridges,
visibile
radiose virtutem suam in
alia
porrigere.
Vol.
II,
pp.
71-72.
337
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DAVID
C.
LINDBERG
But
this does not
get
to the nub of
Ronchi's
argument.
His claim
is
not
so
much that
species
resemble
eidola
in
materiality
as that
species,
like
eidola,
are
coherent
wholes.
Alhazen's
principal
contribution
to
the-theory
of vision
had been to substitute a
point-by-point
analysis
of the visual field
(with
forms
issuing
from
every
point)
for
the
coherent eidolon
of
Epicurean
phi-
losophy,
and
Ronchi's view is that
the
concept
of
species
was
a return to
coherence.
But
this is
surely
not
true
of the
concept
in its
thirteenth-century
(and
most
influential)
form.
Bacon himself felt that the
difference
between
species
and
Alhazen's forms
was
merely
terminological,75
and he was
sub-
stantially
correct:
in
fact,
the
optical
properties
of his "visible
species"
and
Alhazen's
"forms of
light
and
color"
are identical.
Now
on the
question
of
coherence,
Alhazen, Bacon,
and
Pecham
are all
imprecise
in
their
termi-
nology.
Alhazen
frequently speaks
of the
singular
form of
a whole
object,
but this
form
has
the
crucial
property
of
susceptibility
to a
point-by-point
analysis.
Moreover,
when
explaining
the
process
of
sight
or
locating
images
formed
by
reflection or
refraction,
Alhazen
considers the
object
a
point
at
a
time and
appeals
to
individual
rays
instead of
the
unitary
forms. This is
not
unlike
our
practice
of
conceiving
of a
continuous
body
of
radiation
as
a
composite
of
discrete
rays,
emanating
from
individual
points
on the lumi-
nous
body.
On the
question
of
coherence,
Bacon and
Pecham
followed Alhazen's lead
in
every
detail.
Pecham often refers to
the
single
species
of a
whole
object,
explaining, for example, that the "species [singular] produced by a visible
object
has the
essential
property
of
manifesting
the
object
of which it
is
the
likeness,"76
but this
species (like
Alhazen's
form)
is
susceptible
to
puncti-
form
analysis.
On other
occasions,
Pecham
speaks
of
the
species
of
a
point,
remarking,
for
example,
that
"any
point
of an
object
seen
in a
mirror
fills
the
whole surface
of the
mirror
with its
species."77
The
very
foundation
of the
geometrical
and
physiological optics
of
Bacon
and
Pecham is this
ability
of the
visual
field to
be
analyzed
into
points
for
individual
treatment;
their
optics,
like
Alhazen's,
is
based on
rays
(representable by
geometrical
lines)
rather than
a
coherent
species.78
It is
evident, then,
that
Bacon
and
Pecham fully and successfully incorporated in their optics both Alhazen's
75
In De mult.
spec.,
Bacon
says explicitly
that
Alhazen
used the term "form" to denote
species:
"Forma
quidem
vocatur
in
usu
Alha-
zen,
auctoris
Perspectivae vulgatae"
(Bridges,
Vol.
II,
p.
410).
In
his
Questiones
supra
librum
de
causis,
Bacon
attributes
the
concept
of
species
to
Alhazen,
saying,
"Item
secundo
Per-
spective
[i.e.,
Alhazen's]
dicitur
quod
lux
et
colores
multiplicant
suas
species
usque
ad
sensum .
. ."
(ed.
Robert
Steele
and
F.
M.
De-
lorme
in
Opera
hactenus inedita
Rogeri
Baconi,
fasc. XII
[Oxford:
Clarendon
Press,
1935], p.
52).
76".
. .
species
genita
a
re
visibili
essen-
tiali habet rem
ostendere cuius est simili-
tudo
. . "
(Pers.
corn.,
II,
Prop.
5).
77
"
.
.
quilibet
punctus
rei
vise
in
speculo
replet specie
sua
totam
superficiem
speculi"
(Pers.
cor., I,
Prop.
3 of the
revised
version).
In De
mult.
spec.,
Bacon discusses both the
species
(singular) representing
the whole ob-
ject
and the
species (plural)
of
the
individual
parts
in
the
same
argument:
"Hoc
enim
non
est
quia
magnitudo
faciat
suam
speciem,
sed
quia
a tota rei
magnitudine
venit
species
coloris
et
lucis,
et
a tota
superficie.
Et
tunc
species
coloris
venientes a
singulis partibus
rei
visae
non confuduntur in una
parte
pupillae
. . .
(Bridges,
Vol.
II,
p.
429).
78
Bacon
asserts
unequivocally
that
"an
in-
finite
number of
rays
issues
from
every point
of
the
agent"
(Opus
majus,
IV,
Dist.
3,
Ch.
1,
Bridges,
Vol.
I,
p. 122).
338
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ALHAZEN'S THEORY
OF
VISION
punctiform
approach
and
Grosseteste's
concept
of
species.
They
achieved
this
simply by endowing
species
with
all the
optical properties
of Alhazen's
forms.
At
only
one
point
in their theories of vision did Bacon and Pecham find
that
species
were
significantly
at odds with Alhazen's
"forms of
light
and
color."
Species,
according
to
Grosseteste,
issue
from all
natural bodies-from
eyes
as
well as from
perceived
objects;
and
he
argues
that
the
species
issuing
from
the observer's
eye,
as well
as
those
emanating
from
the
object,
play
a
role
in
sight:
Nor
is it
to be
thought
that
the
emission
of
visual
rays
is
only
imagined
and
without
reality,
as
those
think
who
consider
the
part
and
not
the whole.
But
it
should be
understood that
the visible
species
[issuing
from
the
eye]
is
a
substance, shining
and
radiating
like
the
sun,
the
radiation
of
which,
when
joined
with the
radiation
of
the
exterior
shining body,
entirely
com-
pletes
vision.
Wherefore natural
philosophers,
treating
that
which
is natural to
vision
(and
passive),
assert that vision
is
produced by
intromission.
However,
mathe-
maticians
and
physicists,
whose concern is
with those
things
that
are
above
nature,
treating
that which is
above
the
nature
of vision
(and
active),
main-
tain that vision is
produced
by
extramission. Aristotle
clearly
expresses
this
part
of vision
that
occurs
by
extramission
in
the
last
book of
De
animalibus,
saying,
"A
deep
eye
sees from a
distance:
for its
motion
is
neither
divided
nor
destroyed,
but
a visual
power
leaves it and
goes straight
to the
objects
seen." . . . Therefore true perspective is concerned with rays emitted [by
the
eye].79
In
his
Commentary
on
the Posterior
Analytics,
Grosseteste
says
further,
"Vi-
sion is not
completed
solely
in
the
reception
of
the sensible form
without
matter,
but
is
completed
in
the
reception just
mentioned
and
in
the radiant
energy
going
forth from
the
eye."80
Philosophers
of the thirteenth
century
were faced with
the task of
recon-
ciling
Alhazen's denial and
Grosseteste's affirmation
of
the
existence of
visual
rays.
The
situation was
complicated,
of
course,
by
the
presence
of other
treatises
pronouncing
on the
same
subject-the
works
of
al-Kindi,
Euclid,
Ptolemy,
Aristotle,
Avicenna,
and others.
Bacon and
Pecham
felt
they
could
do
justice
to all schools of
thought
by
acknowledging
with Grosseteste that
visual
rays
exist
and, moreover,
are
required
for
sight,
while
ignoring
the
vis-
ual
rays
in the
further
development
of
their
theories
of
sight.81
After
at-
tempting
a rather subtle
reconciliation
of
Aristotle,
Ptolemy,
al-Kindi,
Euclid,
Tideus,
Augustine,
Alhazen, Avicenna,
and
Averroes,
Bacon
concludes,
.
. . the
species
of the
things
of
the world
are not
immediately
suited
of
themselves to
bring
to
completion
an action on
the
eye
because of the
nobility
79
I
have translated this
from
Ludwig
Baur's
double-emanation
theory
of
sight.Latin text in Die
philosophischen
Werke des 80
Quoted
by
Crombie,
Grosseteste,
p.
114.
Robert Grosseteste
(Beitrige
zur
Geschichte
der
81
Relative
to
optics,
Bacon
says,
"I
have
Philosophie
des
Mittelalters, 1912,
9,
pp. 72-73).
determined
not to
imitate one
author;
rather,
The exact reference to
Aristotle is
De
genera-
I
have selected the most
excellent
opinions
tione
animalium,
V, 1,
781a1-10;
this
passage
is
from
each"
(Un
fragment
de
l'Opus
tertium,
obviously
one
of the
sources
of
Grosseteste's ed.
Duhem,
p. 75).
339
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DAVID C.
LINDBERG
of the
latter.
Therefore these
species
must be assisted and
excited
by
the
species
of
the
eye,
which
proceeds
through
the
space
occupied
by
the
visual
pyramid, altering
and
ennobling
the medium and
rendering
it
commensu-
rate with sight. Thus the species of the eye prepares for the approach of the
species
of
the
visible
object
and, moreover,
ennobles the
species
of the
object
so
that
it is
wholly
conformable
to and
commensurate
with the
nobility
of
the
animate
body (i.e.
the
eye).82
Surprisingly, John
Pecham
begins by
apparently
attacking
the
emission
theory.
Following
Alhazen
in
every
detail,
he
goes
so
far
as to
argue,
By
assuming
that
sight
occurs
through
rays
issuing
from the
eye,
mathema-
ticians exert
themselves
unnecessarily.
For the
manner
in
which
vision
occurs
is
adequately
described above
[in
terms
of
intromission],
by
which all
the
phenomena
of
vision
can be saved. Therefore
it is
superfluous
to
posit
such
[visual]
rays.83
But then
Pecham finds that he must make the
appropriate
concessions
to
the
emission
theory
required
by
the
concept
of
species
and
adds,-
with
more hesi-
tation than
Bacon,
The natural
light
of the
eye
contributes to
vision
by
its
radiance. For as
Aristotle
says,
the
eye
is
not
merely
the
recipient
of
action,
but acts
itself
just
as
shining
bodies
do.
Therefore
the
eye
must
have
a
natural
light
in
order
to alter visible
species
and
make
them
commensurate with the
visual
power,
for the
species
are
emitted
by
the
light
of
the
sun and
moderated
with
respect
to the
eye
by
mixing
with the natural
light
of the
eye.
. . . Since vision is of
the same
kind in all
animals,
and certain animals
are
able to
bestow
the
mul-
tiplicative
power
on colors
by
the
light
of their
eyes
so as to
see
them
at
night,
it follows
that
the
light
of the
eye
has
some effect
on
[external]
light.
Whether
it
goes beyond
this,
I do not
determine,
save
only by following
in
the
foot-
steps
of
the
Author
[i.e.,
Alhazen],
as I have
said before.84
Two conclusions
emerge
from
this
analysis
of
Epicurean
eidola,
Alhazen's
forms,
and
Western
species.
First,
neither
Alhazen's forms
nor Western
species
bear
any
but
the most
superficial
resemblance
to
Epicurean
eidola.
82
Opus
majus,
V,
1,
Dist.
7,
Ch.
4,
Bridges,
Vol.
II,
p.
52.
83"Mathematicos
ponentes
visum fieri
per
radios
ab
oculo
micantes
superflue
conari.
Visus
enim sufficienter
fit
per
modum
prescrip-
tum,
per
quem
salvari
possunt
omnia
circa
visum
apparentia.
Ergo
superfluum
est
ponere
sic radios"
(Pers.
corn.,
I,
Prop. 44).
84
"Lumen oculi naturale radiositate sua
visui conferre. Oculus
enim,
ut
dicit
Aristoteles,
non solum
patitur,
sed
agit
quemadmodum
splendida.
Lumen
igitur
naturale necessarium
est oculo ad alterandum
species
visibiles et ef-
ficiendum
proportionatas
virtuti
visive,
quo-
niam
ex
luce
solari diffunduntur sed
ex lumine
oculi
connaturali oculo
contemperantur....
quoniam
visus in
omnibus
animalibus est
unius
rationis cum
igitur
quedam
animalia
per
lu-
men oculorum
suorum sufficiant
coloribus vir-
tutem
multiplicativam
dare
ut
ab eis nocte
videri
possint,
sequitur
ut
lumen oculi
aliquid
in
lumine
operetur.
Et
an
aliquid
ulterius fa-
ciat
non
diffinio
nisi huius
Auctoris,
ut dictum
est,
vestigia
sequendo"
(ibid.,
I,
Prop.
46).
Ba-
con's and
Pecham's admission
that
rays
issue
from
as well
as
enter the
observer's
eye
cannot,
of
course,
be
viewed
solely
as a
compromise
between Grosseteste and
Alhazen.
Bacon and
Pecham were
influenced
also
by
Aristotle and
by
the
Galenic and Platonic
traditions and
should be
considered heirs of these
teachings
as well. As Pecham
points
out in the
passage
quoted,
Aristotle
speaks
of
combined
emission
and
intromission;
and
Pecham's
argument
about
the
visual
power
making
the
visible
species
commensurate with
sight
is
based on
Aristotle's
remarks
in
De
generatione
anima-
lium,
V, 1,
780a5-25.
340
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8/20/2019 Lindberg - Al Hazen’s Theory of Vision and Its Reception in the West (1967)
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ALHAZEN'S
THEORY OF
VISION
Forms and
species
are
powers,
not
corpuscles
or coherent collections
of
cor-
puscles,
and
they
are
susceptible
to
punctiform
analysis.
Second
and more
important,
Alhazen's forms and Western
species
have
identical
optical prop-
erties and
perform
identical
optical
functions-with one
exception.
Against
Alhazen's
denial
of
visual
rays,
Bacon
and
Pecham
admitted
that
species
emanate from the
observer's
eye
and
play
a role
in
sight.
However,
this ad-
mission of
visual
rays
did not
otherwise
alter the
theory
of
vision
gained
from
Alhazen,85
and
Bacon and
Pecham almost
completely
ignored
the
species
emanating
from
the
observer's
eye
in the further
development
of their
theo-
ries. In their
theory,
as
in
Alhazen's,
powers
emanating
from
natural
bodies
produce
effects
in
a
recipient,
and both theories
permit point-by-point
analy-
sis
of
the
visual
field and offer
identical
descriptions
of
the act
of
vision. Thus
the admission of visual rays by Bacon and Pecham was of small consequence
for the
subsequent
history
of
optics.
It
did not
interfere
with
the
transmis-
sion,
through
their
works,
of
the broad
outlines
and
most
of the
details of
Alhazen's
theory
of vision.
Witelo,
whom
I
have
ignored
because of his
superior
status in
Ronchi's
scheme
of
things
and
his
failure
to use the term
"species"
or
discuss
rays
emanating
from the
eye,
shared similar
views
on the
properties
of the
rays
and the
nature of
sight.
After a
careful
study
of
Witelo's
Perspectiva,
Alek-
sander
Birkenmajer
concludes
that
"sous
la
plume
de Witelo le
mot 'forme'
doit etre
identifie
avec
la
species
baconienne . .
.
dans
toute
la
vaste
eten-
due de ce term chez Bacon."86
It is
evident
that
apart
from the
question
of
visual
rays,
Alhazen's
theory
of
vision
and views
on the
nature
of
light
were transmitted
intact to the
West.
I do
not,
of
course,
claim that the
concept
of
species
is
in
all
respects
identical
to Alhazen's
concept
of
form;
the
former,
for
example,
has meta-
physical implications
missing
from the
latter,
and
Vescovini has called
atten-
tion
to subtle
differences
in the
associated
psychologies
of
perception.87 My
point
is
simply
that the
two
concepts
function
identically
as
a
basis for
a
theory
of vision
and
that the
theories
of
vision built
upon
them
by
Bacon,
Pecham,
and Witelo
(on
the
one
hand)
and
Alhazen
(on
the
other)
are
substantially
the
same. Bacon, Pecham, and Witelo initiated a Western
optical
tradition that
faithfully
transmitted
the
essence of
Alhazen's achievement
in
optics
to
Kep-
ler and his
seventeenth-century
contemporaries.
85
Furthermore,
the
species issuing
from the
Bridges,
Vol.
II,
pp.
50-51.
observer's
eye perform
none of the
functions
86
"ltudes sur
Witelo,
II,"
Bulletin inter-
that
Alhazen
denied to
visual
rays
in
his
refu-
national de
l'Academie
polonaise
des
sciences
tation
of the
emission
theory,
as Bacon himself et
des
lettres,
1920,
p.
356.
notes
in the
Opus
majus,
V, 1,
Dist.
7,
Ch.
3,
87
Studi,
Chs.
4,
7.
341