242 CHEMICAL KAYS AND RADIANT HEAT. [MEMOIR XVII. 



ond a golden yellow, and the first uniodized metal ; the 

 object of this arrangement being to expose at the same 

 time and on the same plate a series of films of different 

 colors and of different thicknesses, and to examine the 

 action of the rays impinging on them and the rays re- 

 flected by them. 



Having prepared a second plate, B, and iodized it uni- 

 formly to a yellow, I deposit it in the camera, and now, 

 placing the first plate, A G, so that the rays coming on it 

 from the sky through the window shall be specularly 

 reflected to the object-glass of the camera, and the image 

 of A G form upon B, I allow the exposure to continue 

 until the yellow of A G is beginning to turn brown; 

 then I shut the camera and mercurialize both plates. 



In accordance with what has been said, it will be 

 readily understood that of the bands on A G, the first 

 one, which is the bare metal, does not whiten in the mer- 

 cury vapor; the second, which is yellow, mercurializes 

 powerfully ; the third, which is red, is less affected ; the 

 fourth, which is blue, still less; and the fifth, which is 

 lavender, hardly perceptibly. 



But the changes on B, which have been brought about 

 by the rays reflected from A G, are precisely the con- 

 verse ; the band which is the image of b is mercurialized 

 powerfully ; that of c is untouched and absolutely black, 

 d faintly stained, e whitened, and f mercurialized but lit- 

 tle less than b. 



It follows from this that a white stripe on B corre- 

 sponds to a black one on A G, and the converse ; and for 

 the depth of tint of the intermediate stripes those of the 

 one are perfectly complementary to the corresponding 

 ones of the other. 



By the aid of these results we are now able to give 

 an account of the variability of sensitiveness in photo- 

 genic preparations ; the yellow iodide of silver is exces- 



