040 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 



I may, however, observe in passing, that although I am 

 describing these actions as if thjere were an actual ab- 

 sorption of the rays, and that films on metallic plates ex- 

 hibit colors, 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, corre- 

 sponding marks are obtained, 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 it 

 in a more exact way with this French preparation. Sim- 

 ilar phenomena of interference may be exhibited with 

 the chloride of silver. 



We have it, therefore, in our power to exalt or de- 

 press the sensitiveness of any compound by changing its 

 optical character. Until now, it has been supposed that 

 the amount of change taking place in different bodies, 

 by the action of the rays of light, depends wholly on 

 their chemical constitution, and hence comparisons have 

 been instituted as to the relative sensitiveness of the 

 chlorides, bromides, oxides, and iodides of silver, etc. But 

 it seems the liability to change depends also on other 

 principles which, being liable to variation, the sensitive- 

 ness of a given body varies with them. Thus this very 

 iodide of silver, when in a thin yellow film, is decomposed 

 by the feeblest rays of a taper, and even moonlight acts 

 with energy ; yet simply by altering the thickness of its 

 film it becomes sluggish, blackening even in the sunlight 

 tardily, and recovering its sensitiveness again on recov- 

 ering its yellow hue. 



We have now no difficulty in understanding how, in 

 the preparation of ordinary sensitive paper, great varia- 



