PHOTORECEPTION 177 



(1943) and vigorously criticized by Granit (1947). More recently 

 Antrum (1953, 1958) has proposed a hypothesis which can be summar- 

 ized as follows: Light is absorbed by a photosensitive substance, as a 

 result of which the substance is altered and stimulates the retinal cells. 

 In 'slow* eyes it decomposes, and the photosensitive substance is re- 

 generated. Since these processes require time, adaptation is slow. In 

 *fast' eyes, however, decomposition and regeneration is blocked during 

 illumination, but when illumination ceases the process proceeds very 

 rapidly. In other words, adaptation is very rapid. The blocking is 

 accomplished by the positive potential from the centrifugal cells. 

 This is a very high potential (20 mV as compared with 1 mV in the 

 frog). 



Wulff and his associates (Wulff, Fry, and Linde, 1955; Fry, Wulff, 

 and Brust, 1955) have made a different analysis. Following illumina- 

 tion of the eye there is always a latency before the first wave of the ERG 

 arises. It is assumed that this is the period required for photolysis to 

 initiate the retinal action potential. The coupling processes connecting 

 these two events is obscure. According to the hypothesis of Wulff e/ al.^ 

 there are two processes involved in the sense cells : an electrical pro- 

 cess generating the retinal action potential and an auto-csLtalytic rate 

 process which controls the latent period. Light acting on a photo- 

 sensitive substance generates another substance (C) whose concentra- 

 tion manifests itself as a retinal e.m.f. C is presumed to be generated at 

 a rate proportional to the intensity of light. The retinal e.m.f. is pro- 

 portional to log cone. C. The latency for a flash of light is a hnear 

 function of log / over most of the intensity range tested and is relatively 

 insensitive to time. Wulff e^ ai. suggested that the failure of 'slow' eyes 

 to obey the Bunsen-Roscoe law after 30 msec, may be attributed to 

 decay of an electrically active substance rather than to a recovery 

 process as Autrum (1950) suggested. 



COLOUR VISION 



From the observations of Plateau (1 888) to the present time there have 

 been literally hundreds of recorded observations about insects show- 

 ing preferences for one colour or another. Consequently, there is not 

 the slightest doubt that insects can distinguish among the various 

 coloured objects in nature and that their eyes are sensitive to wave- 

 lengths from about 253 m[x (the near ultra-violet) to about 700 mii (the 

 infra-red). Lubbock (1886) had shown that ants were less apt to move 

 their pupae out of sunlight from which the ultra-violet had been 



