LIGHT AND PIGMENTATION 



91 



it may still be elicited with the more effective stimulus of ultra-violet light 

 after all connections with the central nervous system have been severed. 



(6) The most dramatic reactions follow stimulation of the eyes 

 (SECONDARY RESPONSES). Hogben and Slome (1931), for example, 

 found that in the case of the clawed toad, Xenopus. when the field of 

 vision was occupied by a light-scattering surface the animal became pale, 

 when set in the dark background of a light-absorbing surface the 

 animal suffered generalized darkening of the skin, a response abolished 

 on removal of the eyes. While the primary responses react in general 

 to the total intensity of light, the secondary resjDonse is usually 

 based on the ratio of the amount of incident light entering the eye 

 directly from above to the amount of reflected light from the back- 

 ground, so that on a dark background, when the ratio is large, the 

 animal becomes dark and on a light background, when the ratio is 

 small, it becomes pale ; it is to be noted that these changes occur 

 without regard to the intensity of the total illumination (Sumner, 

 1911-40 ; Sumner and Keys, 1929 ; Brown, 1936 ; Sumner and 

 Doudoroff. 1937). This influence of the reflectance or albedo of the 

 background (to borrow an astronomical term) has been fully established 

 by experiment ; thus the effect of a dark background can be faithfully 

 reproduced by makmg the lower half of the cornea opaque ^ ; reversal 

 of the fish or of the illumination or the background produces the 

 expected effect ; and Butcher (1938), on rotating the eye of the 

 killifish, Fu7idulus. through 180°, found that it gave its tawny response 

 to a yellow backgroimd only when the latter was above the animal. 



In his work on teleostean fishes Sumner (1940) established that the reflect- 

 ance of the substrate had an important effect on morphological colour changes 

 also, for he found that the melanophore count and the total quantity of melanin 

 varied inversely as the logarithm of the reflected light. The mathematical nature 

 of the relationship is interesting and unexpected ; it recalls Fechner's modifica- 

 tion of Weber's Law defining the relationship between the intensity of stimuli 

 and their sensory apjDreciation, and suggests that in assuming control of these 

 colour changes the eye applies the same quantitative standards as govern its 

 sensory activities. 



In addition to this general quantitative reaction, a differentiated 

 response to the siDectral nature of the light reflected from the back- 

 ground is relatively common so that the animal can assume the colour 

 of its environment, sometimes with remarkable rapidity and accuracy. 

 This apparently extraordinary reaction was first scientifically described 

 in the chameleon prawn, Hippolyte, by Keeble and Gamble (1899) and 

 many instances have now received study. The prawn, Paloemon, for 

 exaniiDle, can manipulate its red, yellow and blue pigments, so that with- 



1 In the insert. Di.rippus — Atzler (1930) ; Priebatsch (1933) ; in shrimps — 

 Hanstrom (1937-38) ; in fishes — Sumner (1940). 



Hippolyte 



