426 



Comparative Animal Physiology 



mediate region. The data which indicate atypical trends^^^ do so for tech- 

 nical reasons (see Wulff ^'^*). It is apparent that some of those photoreceptors 

 examined give rise to an electrical response which increases with increasing 

 intensity of illumination over a considerable range. In some cases this in- 

 tensity range is of the order of a million. 



Graham^^ has demonstrated that the electrical response (measured as the 

 time integral of the retinal potential) of the Limulus eye increases under the 

 following conditions: (1) when the number of sensory units (ommatidia) 

 illuminated is kept constant and intensity of illumination increases; (2) when 

 the illumination is kept constant and the number of sensory units illuminated 

 is increased (Fig. 131). These results probably indicate that individual 

 sensory units can produce a graded response. It is probable that the intensity- 

 magnitude data of Figures 129 and 130 represent recruitment of increasing 

 numbers of sensory elements as well as the summed effect of graded re- 

 sponses of the sense elements stimulated. 



^0 m 



Number 



/50 200 



of Ommat/diQ 



290 



Fig. 131. Data illustrating the relation between response magnitude (time integral of 

 the retinal potential change) and number of ommatidia illuminated at four different 

 intensities. The logs of the intensities are indicated on the curves. From Graham." 



ELECTRICAL MEASUREMENT OF DARK ADAPTATION. During dark adaptation 

 a given Hght stimulus becomics more and more effective, i.e., the photochem- 

 ical effect becomes greater. This should be reflected also in the electrical 

 response to a given light stimulus during dark adaptation. The time course 

 of dark adaptation has been followed in several species of animals by meas- 

 urement of the magnitude of the electrical response elicited by a constant- 

 intensity, constant-duration light stimulus. The data obtained by Hartline"'"' 

 from Limxdus and by Rigps^"*- from the frog (2 degree foveal stimulation 

 data), when plotted in terms of log / necessarv to elicit a response of con- 

 stant magnitude as a function of time in the dark (Fig. 116, middle and 

 upper curves), give curves essentially similar to the curve obtained by Hecht 

 (lower curve. Fig. 116— see page 407). 



