Photoreception 



425 



efficients obtained by Hecht and the above values are in fair agreement 

 with the theoretical value of 1.00. 



RECIPROCITY LAW DETERMINED FROM ELECTRICAL MEASUREMENTS OF 



RETINAL ACTIVITY. Numerous measurements^^-- ^'^^' ^"' ^^- ^*' ^'^ relate the mag- 

 nitude of the retinal action potential to the stimulating intensity, employ- 

 ing a constant flash duration (Fig. 129). The data of Hartline, ^'^ presented 

 in Figure 130, are particularly instructive. He has measured the magnitude 

 of the retinal action potential in relation to intensity of illumination, em- 

 ploying diff^erent exposure periods. 



These data reveal that over a wide range the intensity and duration of 

 flash to produce a response of constant magnitude are inversely related, i.e., 

 for longer durations the intensity needed is less, and vice versa. Computa- 

 tion of the intensity-time product necessary to elicit a response of constant 

 magnitude is equal to a constant. Here, then, is a duplication of the applic- 



l£0 



10.0 



6.0 



6.0 



4.0 



£.0 



'f 1 00. =0.060 seconds 



c 

 o 



ex. 





^l^^a-o^^ Log 



5.0 



4.0 



ao 2.0 



.0 



0.0 



Fig. 130. A family of curves relating magnitude of the retinal action potential to 

 log intensity for different periods of exposure. The measurements were obtained from 

 the eye of a grasshopper. From Hartline.** 



ability of the reciprocity law to photoreception, an earlier application 

 having been made by Hecht (see page 406). Again it should be pointed 

 out that the above relation is valid only for constant response levels. The 

 over-all relation between E and It for the photoreceptor of the grasshopper 

 is apparent from inspection of the data in Figure 130, and is the same as the 

 relation between magnitude of the retinal action potential and logarithm of 

 the intensity. 



RELATION BETWEEN INTENSITY AND MAGNITUDE OF ELECTRIC RESPONSE. 



Inspection of the curves in Figure 129 and in Figure 130 indicates that the 

 relation between intensitv of the light stimulus and the magnitude of the 

 retinal action potential is a rather consistent one. The curves are sigmoid, 

 with a plateau at the high and low intensity ends and a rather linear inter- 



