COLOR PHOTOGRAPHY. 199 



brassicw and of Pieris rapce, what radiations of the spectrum are most 

 active in decomposing the dark pigment of the epidermis. The results 

 were exhibited in a plot 1 whose abscissae corresponded to the color of 

 the illumination and whose ordinates gave the estimated degree of 

 attack upon the dark coloring matter of the epidermis. Besides the 

 already-mentioned maximum of decomposition by the action of orange- 

 colored light of wave length between 570 and 650 jj/j, he found in the 

 case of Pieris rapw a second, though less marked for bright-green light 

 with wave lengths between 510 and 584 fxjj.. It is particularly the yel- 

 low constituent of the light sent out by green leaves which is able to 

 destroy the dark coloring matter most effectually. The extreme red 

 and blue portions of the spectrum are scarcely more active than 

 darkness. 



The similarity with the processes of poly-color photography goes even 

 further. In the epidermis of green caterpillars of Amphidasis betu- 

 laria, which is able to secrete the dark coloring matter, Ponlton found 

 instead of this a pale yellow coloring matter, which had a greenish- 

 yellow appearance under the microscope. " It is therefore clear that 

 the surroundings determine not only the presence or absence of true 

 pigment in the epidermic ceils, but also its constitution, and therefore 

 color when present.'" 



The green coloring matter in the fatty layer can be partially de- 

 stroyed, 2 for example, in white light. Therefore, it also receives rays 

 which it absorbs and therefore act upon it. 



A test of the explanation given is formed for color photography by 

 the crossed spectra experiment. Poulton arranged a similar experi- 

 ment. He moved caterpillars from dark to light surroundings, and 

 vice versa, an experiment which he designates " transference experi- 

 ment." 3 He found that a change of the first color in a sense such as 

 to cause it to approach the second was to be noticed so long as it was 

 confined to the time during which the caterpillar was sensitive. Here, 

 however, occurs a great difficulty to the understanding of the phe- 

 nomenon, which I must mention in detail. 



It must first of all be remarked that in the previously mentioned 

 phenomena of the skin of caterpillars it appeared as if it secreted a 

 coloring matter which in the periods of sensitiveness possessed in some 

 degree the character of a color- receptive substance. But in order that 

 it could be said that the caterpillar's skin behaved like a photographic 

 plate, it mast be shown that two different parts of the skin which were 

 subjected to different colored illumination assumed different colors. 



One such observation has been made, but it appears to be the only 

 one. It was communicated by Mrs. Barber in a memoir which was 

 laid before the Entomological Society of London by Darwin. 4 A pupa 



1 Poulton. Phil. Trans., 178, fig. 6, page 431. 1887. 



2 Poulton. Trans. Ent. Soc, page 359. 1892. 



3 See for example Trans. Ent. Soc., pages 352, 419. 1892. 

 'Ent. Soc. Trans., page 519. 1874, according to Ponlton. 



