NATURAL PHILOSOPHY. 207 



the color of the prepared plate, without causing it to lose all trace of chlo- 

 rine, but at the same time it changes the mode of action of the various 

 luminous rays. The diffused or direct solar light acts as white, instead of 

 giving an impression of a gray color ; and besides, the colored tints are light 

 instead of dark, as before the operation. But the most remarkable part 

 is, that, by keeping the temperature up to 8G 1\ or 97 F. for several 

 d.iys, the same end is attained, and with even better results. The yellow 

 and green colors, which, except the action of the luminous spectrum at a 

 high temperature on a plate which has been operated on, are not produced 

 with clearness, appear in these conditions ; moreover, the sensitive matter 

 is more rapidly sensitive. The plates thus prepared may likewise be ren- 

 dered useful for the reproduction of colored images in the camera- obscur a. 



We cannot attribute to a chemical action the effect produced on chloride 

 of silver by a difference of temperature which is so slight, but maintained 

 for several days. It is probable that in these circumstances a modification 

 of the physical state of the sensitive substance takes place. This would 

 then be an effect of the same kind as that which takes place in the forma- 

 tion of red phosphorus. 



The action of the least refrangible rays of light is likewise very curious, 

 for it leads to a result analogous to that obtained by prolonging the eleva- 

 tion of temperature on the plates. It appears, then, that in both cases 

 molecular effects of the same order are produced. The luminous spectrum 

 acts in the following manner on the chloride of silver, modified by the 

 extreme red rays. The action begins as before with the orange, yellow, 

 and green, then extends gradually towards the violet and red. All the 

 tints corresponding to the colors of the spectrum are as bright as if the 

 plates were annealed, but the prismatic impression is far more beautiful ; 

 and even the green, the yellow, and the orange have more brilliant tints 

 than before the action of the extreme red rays. Therefore, to the advantage 

 which is possessed by the chloride modified by the least refrangible rays 

 over that which has been annealed, of giving the black ground upon which 

 the different prismatic tints are depicted, is added that of preserving per- 

 fectly the yellow and green tints. On the red side, the image of the spec- 

 trum is brilliant only as far as B ; beyond that limit, the black tint which 

 is produced being the dominant one, no effect takes place during the first 

 few moments. However, if in the first place the chloride has remained 

 an insufficient time under the action of the extreme red rays, tl 9 solar 

 spectrum still gives a dark impression beyond B, and even A. 



We obtain with the matter thus modified by heat or by light very beau- 

 tiful colored reproductions of the luminous spectrum. The figures of 

 colored rings, and those given by crystallized plates traversed by polarized 

 light, are likewise well represented in their various tints. We can like- 

 wise reproduce the images of the camera- obscura, which are, so to speak, 

 painted by the light ; but these reproductions, although brighter in color 

 than any I have obtained before, have as yet only a scientific interest, and 

 I cannot, as yet, hope for any application of them, as the impressions are 



