Photoreception 



417 



(retinenci and protein) at the beginning of dark adaptation. This equation 

 describes quite well all of the recovery curves of the family obtained after 

 different amounts of light adaptation. Data^- obtained from the photore- 

 ceptor of Linmlm (p. 426) indicates a family of recovery curves after dif- 

 ferent degrees of light adaptation rather than the single curve obtained earlier 

 and described (p. 407). 



LOG INTENSITY (I) 



Fig. 125. Diagram showing the rod and cone curves for visual acuity plotted against 

 log intensitv-. The insert (upper left) indicates the type of visual field used in these 

 experiments. From Jahn.^°" 



Visual acuity. By visual acuity we mean the abilitv to see small ob- 

 jects. One simple form of experiment is shown in the insert of Figure 125. 

 The observer is confronted with a field of intensity 7 and a black line of 

 variable width, and he is allowed to determine the width of the line which 

 can just barely be seen at various intensities. Results of a typical experiment 

 are shown in the curve of Figure 125, where the reciprocal of the width is 

 plotted against intensity. Because of diffraction the image on the retina for 

 the narrowest line that can be seen is not in black and white, but in gray 

 and white. Therefore, this experiment is reallv one in brightness discrimina- 

 tion, and anv explanation for brightness discrimination should also explain 

 visual acuity. 



Photochemical Explanation. The data for the above tvpes of experiments 

 can be predicted from a knowledge of the photochemical cycle. The theory 

 and equations for that purpose outlined below are those of Jahn "^•'^''' '^^ 

 These differ somewhat from the earlier formulations of Hecht"^' "•'^' '^^- '^^- "^^ 

 and from those of other investigators.^^- ^^- ^^*' 



The fundamental assumption is that the threshold (or any other given 

 level) of sensitivity is equivalent to some fixed value for the rate of the 

 LT reaction, i.e., for dT/dt. The factors which affect this rate are the con- 

 centration of the substrate L, the concentration of the enzvme E, and a 

 velocity constant which varies with temperature. In the case of flicker it is 

 assumed that the value of the maximal critical frequency is determined not 



