500 F. W. (lAMJU/K. 



In view of this very decided coniplementary colour-change 

 the regrettable mortality that occurred in vessels exposed to 

 green light in 1908 does not seriously diminish the value of 

 this result^ though larger numbers would add to its cogency. 

 These were obtained in 1909. The experiments of 1908 and 

 1909 are compared on Table V and with the other experiments 

 of this paper on Table IV. 



(2) The Influence of White Backgrounds and of 

 INloiioch rouia tic Back "-rounds in \Vhite Litrlit. 



The effect of short exposures to the influence of white 

 (porcelain) and of l)]ack (cloth or ])aint on glass) backgi-ounds 

 on the colouring of young and old Hippolyle has been fully 

 treated in a previous menioir.' It was there shown that what- 

 ever the quality or ([uantity of the light employed (within the 

 experimental limits), the background effect dominated, pro- 

 ducing contraction if white and expansion if black. It 

 occurred to me, however, to see whether the same results 

 would follow a long exposure made with young specimens in 

 which tlie pigments were rapidly developing. 



The results of a nionth's trial are of considerable interest. 

 The Hi})polyte on black surfaces simply followed the usual 

 procedure under such conditions, and developed maximal 

 amounts of red and yellow pigments, which gave them a deep 

 reddish tint. On the white background, however, after a 

 first phase of transparency, they began to develop red pigment 

 along- the nerve-cord, and finally became uniforndy marked 

 with a ventral red stripe, whilst over the rest of the body the 

 pigments were reduced to microscopic dots or disappeared. 

 This remarkably adaptive result was obtained in diffuse light, 

 the top of the deep porcelain vessel being covered with muslin, 

 through which a stream of water was maintained from a tank 

 above (Table IV, p. 579). 



In 1909 these background effects were extended so as to 

 include the results of red and green. The vessels employed 

 were large museum jars, painted, except for a large rectangular 



1 (1904), p. 353. 



