416 



Comparative Animal Physiology 



The several products resulting from the bleaching of rhodopsin, i.e., vis- 

 ual yellow and visual white, indicate that the kinetics of the production of 

 rhodopsin is not as simple as was originally formulated by Hecht (page 408). 



Instead of a single reaction, there are now known to be three reactions 

 involved in the synthesis (Fig. 119). This complexity of the reaction is in 

 evidence if a series of dark-adaptation curves obtained from one photore- 

 ceptor, as in Figure 124, are compared. The curve of recovery after adapta- 

 tion to a low intensity of illumination, e.g., 4 millilamberts, is quite different 

 from that of recovery after adaptation to high intensity of illumination, e.g., 

 4700 millilamberts. Recovery curves obtained after intermediate intensities 

 of illumination form a graded series between the two extremes. Jahn^'^ dem- 

 onstrated these differences by plotting the data as shown in Figure 124, 

 rather than in the conventional log of threshold intensity-time plot used by 

 most workers. 

 1 



Fig. 124. Curves showing the progress and extent of dark adaptation after exposure 

 to Hghts of different intensity. Data from a normal human subject. The ordinate repre- 

 sents the reciprocal of threshold intensity, the abscissa represents time (minutes) in the 

 dark. The numbers on the curves indicate the intensity (millilamberts) of the adapting 

 light. From Jahn.*° 



Inspection of this series of curves (Fig. 124) revealed that the data for 

 the lowest adapting intensity could be fitted best by the equation for a mono- 

 molecular reaction (p. 407), whereas the curve for the highest adapting 

 intensity could be fitted with the equation for a monomolecular auto- 

 catalytic equation. The intermediate curves could not be fitted by either 

 of these two equations. Jahn^" proposed an equation relating the 

 increase of rhodopsin (Z) with respect to time in the dark (t), 



1 t + a 

 Z= 1 



t + a t + a + 1 + Ket 



where a denotes the reciprocal of the initial concentration of visual yellow 



