the Chemical Rays and those of Radiant Heat. 209 



examine the action of the rays impinging on them, and the 

 rays reflected by them. 



Having prepared a second plate, B, and iodized it uniformly 

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

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

 window from the sky shall be specularly reflected to the ob- 

 ject-glass of the camera, and the image of A G form upon B, 

 I allow the exposure to continue until the yellow of AG is 

 beginning to turn, brown ; then I shut the camera and mer- 

 curialize both plates. 



In consequence of what has been said (58.), 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 mercury vapour ; 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 perceptible. 



But the changes on B, which have been brought about by 

 the rays reflected from A G, are precisely the converse ; the 

 band, which is the image of 6, is mercurialized powerfully; 

 that of c is untouched and absolutely black, d faintly 

 stained, e whitened, and f mercurialized, but little less 

 than b. 



(65.) 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 ac- 

 count of the variability of sensitiveness in photogenic prepara- 

 tions ; the yellow iodide of silver is excessively sensitive, be- 

 cause it absorbs all the chemical rays that can disturb it, 

 whilst the lavender is insensitive, because it reflects them. 

 Under this point of view, sensitiveness therefore is directly as 

 absorption and inversely as reflexion. 



The superiority of Daguerre's preparation over common 

 sensitive paper may now be readily understood. It absorbs 

 all the rays that can affect it, but the chloride of silver, spread 

 upon paper, reflects many of the active rays. The former, 

 when placed in the camera, gives rise to no reflexions that 

 can be injurious; the latter fills it with active light, and stains 

 the proof all over. Hence the Daguerreotype has a sharp- 

 ness and mathematical accuracy about its lines, and a depth 

 in its shadows, which is unapproachable by the other. More- 

 over, the translucency of the white chloride of silver, as well 

 as its high reflecting power, permits of particles lying out of 



Phil. Mag. S. 3. Vol. 19. No. 123. Sept. 18*1. ' P 



