620 F. W. GAMBLE AND F. W. KEEBLE. 



lutely pure monocliromatic lights but the quantity of light 

 other than that of the colour desired is not considerable, and 

 has been accurately determined by Landolt. 



In none of our experiments did we obtain evidence that the 

 coloured light exerts, by virtue of its quality, any positive 

 influence on the diurnal conditions of the pigments. Even 

 when bright light of high intensity was reflected on to the 

 filters by means of mirrors, the results were exactly those 

 which we obtain by subjecting the animals to very dim light 

 or darkness. The prawns exposed to approximately pure 

 red or blue light rapidly became of their nocturnal colour 

 (Sect. V). Green light has, however, less effect. 



Table VI, p. 678, gives the result of experiments with differ- 

 ent coloured lights compared with those made by varying 

 the light intensity. Specimen A, a light brown Hippolyte, 

 in which the red and yellow pigments were well expanded, 

 was put at 12.20 under a red colour-screen, through which 

 daylight- was transmitted. At 1.5 the pigmentary condition 

 had become entirely altered. The red and yellow were 

 retracted and replaced by blue. As far as the colour was 

 concerned the prawn was now a " nocturne " (as we explain 

 in the next section) . The same change occurred in another 

 specimen (D) placed in blue light. Those in the green light 

 (B and C) had up to 1.5 shown no change; while the brown 

 prawns (Gr and H) in white porcelain dish had become green, 

 and one on grey-black sand (I) from brown had changed to 

 an even darker tint. At 2.30 Gr and H were transferred 

 to a glass jar standing on a black plate, where they became 

 and remained dark brown. At 7.5 p.m. all the prawns in 

 the coloured lights were blue ('' nocturnes "), those in the 

 porcelain dishes were greenish, and the others, on the black 

 surface, brown. Our other experiments confirm this result, 

 that coloured lights, if they produce any effect, cause a 

 retraction of the red and yellow pigment and expansion of 

 the blue, accompanied by increased transparency; in fact, 

 the result is the blue colournormally induced by the darkness 

 of night (see Summary of Experiments on p. 652). 



