MKMOIK XVII.] CHEMICAL KAYS AND RADIANT HEAT. 239 



indeed, that the first yellow alone is sensitive to light. 

 We shall see in a few moments that this is very far 

 from being the case. 



Having brought nine different plates to the different 

 colors just indicated, I received in the camera on each 

 the image of a uniform gas-flame, treating all as nearly 

 alike as the case permitted. I readily found that in 

 No. 1 there was a well-marked action, No. 2 still stronger, 

 but that the rays had less and less influence down to 

 No. 6, in which they appeared to be almost without 

 action ; but in No. 7 they had recovered their original 

 power,' being as energetic as in No. 2, and from that 

 declining again. This is shown in Fig. 38. 



Yellow. Blue. Lavender. 2d Yellow. 



No. 2. No. 4. No. 6. No. 7. 



Fig. 38. 



Hence we see that the sensitiveness of the iodide of 

 silver is by no means constant; that it observes period- 

 ical changes depending on the optical qualities of the 

 film and not on its chemical composition; and that by 

 bringing the iodide into those circumstances that it re- 

 flects the blue rays we greatly reduce its sensitiveness, 

 and still more so when we adjust its thickness so as to 

 give it a gray metallic aspect. But the moment we go 

 beyond this, and restore by an increased thickness its 

 original color, we restore also its sensitiveness. Here, 

 then, in this remarkable result we again perceive a cor- 

 roboration of our first proposition. 



