COLOR PHOTOGRAPHY. 203 



be the direct result of differences between the two regions in respect 

 to temperature, light, etc." 1 



It must be taken into consideration that our judgment upon the 

 degree of color adaptation is disturbed by the insensitiveness of our 

 eyes to the extreme violet and the ultraviolet rays ou the one baud 

 and those of the infrared on the other. These cause, however, chiefly 

 blackening and must, therefore, be avoided. 2 



In this connection the following experiment deserves consideration, 

 which I performed with Poitevin plates. These were brighter when 

 the ultraviolet rays were removed from the undecomposed electric 

 light employed in illumination by an absorbing solution, and, on the 

 other hand, darker when these were suffered to pass through unhin- 

 dered. This is a consequence of the decomposition of the silver proto- 

 chloride in the first instance and of its new formation in the second. 

 In similar experiments I recognized the darkening effect of heating 

 and the favorable changes induced by moisture. 



Finally, there is at least to be recognized a relation between the 

 color adaptation of caterpillars and color photography in so far that 

 the caterpillars secrete a coloring matter which to a certain degree 

 possesses the characteristics of a color-receptive substance. In this 

 sense the color adaptation of a single caterpillar must be regarded as 

 mechanical. This is, however, not in contradiction to the conception 

 that the capacity is attained by biological adaptation in the sense of 

 Darwin, for those individuals will be best protected whose pigment 

 is most color receptive. 



It can not easily be decided whether this capacity is developed by the 

 action of light, according to Roux and Eimer, :! or only by chance alter- 

 ations of the protoplasm in the course of time, according to Weismann. 

 It must, however, be remembered that there are no completely fast 

 coloring matters, and that all would to a certain degree be color 

 receptive. Thus the early ancestors of the caterpillars, while not pos- 

 sessing the color adaptation of the present representatives, would 

 still be somewhat changed by light. According to Eimer, it must be 

 supposed that this chemical change would not be without influence on 

 the constitution of the protoplasm and of posterity, and thus their 

 individual changes would receive a certain directing impluse. These 

 changes would, therefore, not require an accidental impression. But 

 even if Elmer's hypothesis should be untenable such an accidental 

 impression would be regarded in a physical sense only as an example 

 of the play of unknown processes which still require explanation. 



•Mr. O. Wackerzapp gave rue tire privilege of examining a series of butterflies in 

 Iris rich collection for which the influence of the region, or the climate, as, for exam- 

 ple, on the north and south sides of the Alps, or the elevation, was distinctly vrsrble in 

 the gradations of color. The else insignificant variations are scarcely to he under- 

 stood except as they are to he ascribed as the effect of light, heat, and other impulses. 



2 Zenker. Photochromie, page 59. 



3 Roux and Eimer. See citations, pages 19,21. 



