THE BEFiATION BETWEEN^ LIGHT AND PIGMENT-FORMATION. 569 



the pigments developed in Hippoly te, when kept in diffused 

 monochromatic light, are not the same as those which appear 

 in specimens kept in daylight on a background reflecting 

 these rays. On a red background in Avhite light, Hippolyte 

 becomes reddish-orange; in pure red light it becomes 

 yellowish or green. On a green background in white light 

 Hippolyte becomes pale green. In pure green light it 

 becomes crimson. On backgrounds of weeds, young colour- 

 less specimens speedily acquire the corresponding tint. Mono- 

 chromatic light, then, when saturated, has an entirely different 

 effect from the same light diluted with daylight. As we pass 

 from the surface to the deeper waters of the sea this dikition 

 becomes less marked. The "background effect," so potent 

 in producing the more littoral colour varieties, becomes less 

 overwhelming as the red and yellow rays are absorbed by the 

 surface waters. Further down, in British coastal waters, the 

 blue end of the spectrum is said to be absorbed, so that at 

 eight fathoms the dominant light rays are greenish or 

 bluish-green (Oltmanns^). Consequently the effect of satu- 

 rated monochromatic light is most probably felt in the region 

 below the eight-fathom line. 



If this distinction between the effects of coloured back- 

 grounds in white light and of diffused monochromatic light 

 on pigment production is Avell founded, it should be supported 

 by analogous results in other animals. Fortunately the work 

 by Poultou and others upon Lepidopterous pupa3 give a 

 closely comparative result. As will be seen from the appended 

 table extracted from their papers, the effects of monochromatic 

 light are very different according as to whether the dominant 

 rays are or are not diluted by white light. Although these 

 experiments have not been made with a view to excluding 

 background results so completely as those given in this 

 paper, yet the distinction between the effect of red light, for 

 example, when concentrated and when diluted, is quite 

 analogous in the case of larval pigmentation in insects to its 

 effect on pigment-production in Crustacea. As a pure 



1 • Jahrbuch. Wiss. Botanik.,' 1892, p. 420. 



