Photoreception 



419 



tegrals (i.e., ogive curves) in log I. The fundamental assumption is that the 

 controlling mechanism in these phenomena is the statistical distribution of 

 thresholds (in log 7). At low intensities only the most sensitive elements 

 (rods or cones and associated neurones) respond; at high intensities all of 

 them respond. Dark adaptation curves as a function of time are explained 

 as the progressive increase of sensitivity of individual units and may be fitted 



Fig. 126. Data showing the relation of the flicker fusion frequency to the log of the 

 intensity for man (uppermost curve) and the turtle (lower curves). Note the shift to the 

 right in the curve for Pseiidemys produced by a lower temperature. The curves were 

 calculated from equations. From Jahn."* 



if the sensitivity is treated as a probability integral in log of time in the 

 dark. On the basis of this theory the assumed distribution of thresholds cov- 

 ers at least four and sometimes as manv as eight log units. The thresholds 

 are supposed to vary from element to element and from moment to moment. 

 If one accepts these postulates, all of the data can be explained. 



This theory, of course, is a source of frustration to investigators who wish 

 to reveal the mechanism of either the sense cells or associated neuronal net- 



