Jacob Riis exposed the ills of the Industrial Age by taking pictures of tenements in New York City. Since there was barely any light in the tenements he created an early system of flash. He would fire a pistol loaded with cartridges of magnesium and potassium chlorate that would create a white light when fired. He published his photographs in a book called How the Other Half Lives. This book convinced middle and upper class people to take part in improving society. Jacob Riis also exposed the effectiveness of the police in New York City. He was hired by Teddy Roosevelt to take pictures of policemen at their post at night. He was able to capture these pictures with his flash device. His investigation led to the discovery that 9 out of 10 policemen werent at their posts. After these pictures were printed in the newspaper, more policemen were at their posts.

Jacob Riis was able to expose the poor living conditions of the impoverished people of New York and was also able to increase the effectiveness of police through photography and flash.