120 ANNUAL OF SCIENTIFIC DISCOVERT. 



HELIOCHRO3IY. 



It is well known that M. Niepce Saint- Victor of Paris, has for a long 

 time occupied himself with the very interesting subject of the repro- 

 duction of colors by photography. Some time since he announced to 

 the scientific world his success in obtaining red, blue, and green ; but, 

 at the same time, he confessed that to obtain a yellow tint in combina- 

 tion with others was a matter of extreme difficulty, if not, at that time, 

 practically impossible to him. Of course, there was nothing at all sur- 

 prising in this, as every one knows that yellow is most troublesome even 

 in ordinary photography. Within the past year, however, he has an- 

 nounced to the French Academy, that he has at last succeeded in re- 

 producing yellow tints by preparing his silver plates in a bath composed 

 of hyperchloride of soda instead of potash, and he produced specimens 

 which are said to hold out great expectation of complete success. He 

 has not yet, however, succeeded in absolutely fixing the colors ; they 

 remain perfect so long as the plate is kept in the dark, but soon disap- 

 pear when exposed to the light. But in this respect, also, M. Niepce 

 has made important progress; for, by the application of gum benjamin 

 as a varnish to the plate, he has managed to retain- the colors for three 

 or four days even when exposed to the full glare of a July sun. The 

 recent memoir read before the Academy by M. Niepce contains much 

 interesting matter. Amongst other things, he has discovered that all 

 compound colors are decomposed by the heliochromic process. The 

 examples given are highly interesting, for instance, if a natural green, 

 such as that of the emerald, of arsenite of copper, of oxide of chromium, 

 sulphate of nickel, or carbonate of copper, be presented, it is repro- 

 duced on the plate ; but, if the green be a compound formed, for in- 

 stance, of chrome yellow and Prussian blue, that of a textile fabric dyed 

 with a mixture of the two latter colors, or that produced on glass in a 

 similar manner, it produces a blue color in whatever manner it is treated. 

 Moreover, when transparent blue and yellow glasses are used, so as to 

 produce a green, it matters not whether the blue be before -or behind 

 or placed between two glasses of the other color, the effect is invari- 

 ably the same ; no matter how long they are exposed to the light, the 

 product is always blue. An orange effect produced by the combination 

 of red and yellow glasses produces invariably red. A red and blue 

 glass together produce at first a violet, because the plate itself is red ; 

 but the result is blue. White paper colored green by means of the 

 recently-discovered Chinese green, made from the juice of the buck- 

 thorn, has but a sluggish action upon the heliochromic plate ; but, after 

 a long exposure to the light, a blue-gray is produced ; and the same ef- 

 fect is obtained from foliage of a grass-green color in the camera ; but 

 bluish-green foliage, such as that of the leaves of the dahlia, produces 

 a tint that is almost positive blue. The eye of a peacock's feather is 

 well rendered in the camera, the tints appearing to vary between 

 blue and green. Apart from photographic purposes, the experiments 

 of M. Niepce Saint- Victor promise to be of considerable assistance in 

 the analysis of the solar spectrum, for it is evident that his attempts to 

 fix the colors of nature on a heliochromic plate go far to confirm the 

 new theory which recognizes the existence of three, but not of the seven 

 primitive colors; namely, violet, indigo, blue, green, yellow, orange 

 and red. 



