402 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 
amounts of sensibility which the surface of a plate has reached by the fii-st exposure 
could be represented by depths of colouring, the various shades would represent the 
picture itself. With a subsequent uniform insolation the chemical action must there- 
fore proceed proportionally to the various amounts of light in the original picture, and 
if the chemical action is made visible by a change of colom:, an actual picture must be 
produced. 
Having determined in this part of our investigation the most important phenomena 
of photo-chemical induction, we shall in the next Part consider the laws which regu- 
late the chemical action of light after the induction is completed. 
