Photoreception 



427 



There may be some question why these dark-adaptation velocity meas- 

 urements conform to the bimolecular isotherm, in view of the complexity 

 of the rhodopsin cycle previously discussed (see page 411). As indicated 

 below, it is apparent that these measurements do not reveal the complex re- 

 lations which actually obtain. In the measurements on frogs,''*- attention 

 is directed to the fact that the data represent cone adaptation rather than 

 rod adaptation. Electrical measurements of dark adaptation in the albino 

 rat-' conform much more closely to the reaction rate of a monomolecular re- 

 action than to a bimolecular reaction rate. In this respect the data of Char- 

 pentier agree with the data of Tansley,'-'^^ who followed dark adaptation in 

 the albino rat by extraction of rhodopsin. Tansley demonstrates that the 

 latter data are fitted best by the velocity equation for a monomolecular re- 



Fig. 132. Retinal action potentials obtained from a beetle, Dytiscus marginatus, during 

 a sojourn in the dark for 4 days. Note the persistent change in the polarity of the 

 response twice every 24 hour period. From Jahn & Wulff.^°* 



action. There is rather good correspondence between the electrical and chem- 

 ical measurements of dark adaptation in the albino rat. Chase and Smith-' 

 measured the time course of regeneration of an in vitro solution of rhodop- 

 sin extracted from frog retinae. The data are fitted by the monomolecular 

 equation. Hence, the measurements of dark adaptation velocities as a whole 

 afford considerable inconsistency. More recently the electric response of the 

 human eye has been measured during dark adaptation.'^-''" 



OTHER CONTRIBUTIONS FROM MEASUREMENT OF ELECTRICAL ACTIVITi' 



OF PHOTORECEPTORS. At least several genera of beetles (^Hydrous, Chlaenius, 

 Harpalns, Dytiscus^ exhibit an inherent diurnal rhythm in the type of 

 electrical response of the eye.'"'^- '"*• '"^- '^"^ Figure 132 shows the types of 

 records obtained from Dytiscus in response to a constant-intensity, constant- 

 duration test flash at various times throughout a 4-day period. Until the 



