the Chemical Bays and those of Radiant Heat. 207 



most sensitive, and photogenic draughtsmen generally suppose 

 that (he others are less efficient from the circumstance of the 

 film of iodide being too thick. Some suppose, 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. 



(58.) Having brought nine different plates to the different 

 colours just indicated, I received on each the image of an 

 uniform gas-flame in the camera, 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. 6. 



(59.) Hence we see, that the sensitiveness of the iodide of 

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

 changes which depend on the optical qualities of the film, 

 and not on its chemical composition ; and that by bringing the 

 iodide into those circumstances that it reflects 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 in- 

 creased thickness its original colour, we restore also its sensi- 

 tiveness. Here then, in this remarkable result, we again per- 

 ceive a corroboration of our first proposition. 



(60.) I may, however, observe in passing, that although 

 I am describing these actions as if there was an actual ab- 

 sorption of the rays, and that films on metallic plates exhibit 

 colours, not through any mechanism like interference, but 

 simply because they have the power of absorbing this or that 

 ray, there is no difficulty in translating these observations 

 into the language of that hypothesis. When the diffracted 

 fringes given by a hair or wire in a cone of diverging light 

 are received on these plates, corresponding marks are ob- 

 tained, a dark stripe occupying the place of a yellow fringe, 

 and a white that of a blue. I found, more than four years ago, 

 that this held in the case of bromide of silver paper, and have 

 since verified in a more exact way with this French prepara- 

 tion. Similar phaenomena of interference may be exhibited 

 with the chloride of silver. 



(61.) We have it therefore in our power to exalt or de- 

 press the sensitiveness of any compound, by changing its 

 optical conditions. Until now, it has been supposed that the 

 amount of change taking place in different bodies, by the 

 action of the rays of light, depended wholly on their chemical 

 constitution, and hence comparisons have been instituted, as 



