72 THE VERTEBRATE RETINA 



The kink is often sharper than we might expect it to be, if it repre- 

 sents a transition. It is accentuated — that is, the overlap of rod-func- 

 tioning into the physiological realm of the cones is reduced — by little- 

 understood phenomena of mutual inhibition of rods and cones. Circum- 

 stances which favor one of the mechanisms allow it somehow to sup- 

 press, partially, the activity of the other mechanism. Thus the rods or 

 cones of a 'pure' retina in some ways exceed in performance their 

 counterparts in a duplex retina. 



When the rate of flashing of an intermittent light is speeded up, a 

 point is reached at which the successive impressions fuse and the light 

 appears to burn steadily. This 'critical frequency of fusion for flicker' 

 has been much studied in man and animals — in the latter by indirect 

 methods, of course, involving training or the recording of the electrical 

 discharges from the retina. The critical frequency increases with inten- 

 sity (strictly, with the logarithm of intensity — = Ferry-Porter law) . At 

 an intensity of 0.25lux— the cone threshold — the critical-frequency 

 curve of a duplex retina such as the human shows a kink (Fig. 27). 

 When colored lights are used, the effect of color on the critical frequency 

 begins to manifest itself only above the cone threshold, as would be 

 expected. With red light, there is no kink— the rods being insensitive to 

 deep red, however intense. Only the cone part of the flicker-fusion curve 

 is obtained from foveal stimulation; and, the farther peripherally the 

 area stimulated, the closer the whole curve simulates that part due to 

 the rods alone. A pure-cone retina, such as that of a turtle, gives a kink- 

 less curve. The pure-rod gecko has also been found to give a homogen- 

 eous curve — though the curve is that characteristic of cones, which 

 seems surprising until one takes into account the fact that the geckoes' 

 rods were secondarily derived from cones (see Fig. 25) . 



Another visual phenomenon which plots a kinked curve is the thresh- 

 old of intensity discrimination. By this is meant the proportion by which 

 a light must be increased in intensity in order for it to be seen to have 

 brightened. The initial intensity being designated "I", the increment is 

 "dl". The curve of "I/dl" plotted against "I" (Fig. 27) shows a change 

 of slope, or kink, at the cone-threshold intensity. With only foveal 

 stimulation there is again no kink; nor is the rod part of the curve, or 

 any kink, obtained with red light. 



Perfectly familiar to all is the increase of visual acuity with intensity 

 — so very commonly do we speak of a light as being "not bright enough 

 to read by." Less apparent is the existence of a kink in this relationship 



