76 THE VISUAL PROCESS 



turn can be made to yield Visual yellow' and 'indicator yellow'. Certain 

 of these photosensitive substances have previously been identified as 

 partial-breakdown products of rhodopsin when it is struck by light. 



The most important properties of rhodopsin are its intense colored- 

 ness, its sensitivity to all visible wavelengths excepting those deep red 

 ones which (by reflection from it) give it its own color, and the fact 

 that its response to these wavelengths is to disrupt into colorless or pallid 

 substances of little or no photosensitivity. It is most affected by the blue- 

 green region of the spectrum, centering at about A500m|l. One might 

 expect that this wavelength would appear brightest to the dark-adapted 

 eye in which rhodopsin has built up to a high concentration. Owing how- 

 ever to modifying factors (chief of which is believed to be the high 

 absorption of short-wave light in the ocular media), the brightest point 

 in the scotopic spectrum is shifted red- ward, to ?L510m[i. One of the two 

 or more substances into which rhodopsin is broken down by light is 

 presumed to irritate the protoplasm of the rod and cause a wave of 

 electrochemical activity, much like the impulses which flow along nerve 

 fibers, to pass down the rod foot-piece and stimulate the bipolar neuron. 



Dark Adaptation — Rhodopsin is not as all-important as it was once 

 thought to be, but it is largely responsible for the ability of the rod to 

 'dark-adapt' or lower its threshold — until the amount of light needed to 

 stimulate it is a tiny part of that required to arouse a cone. While we are 

 in ordinary daylight there is believed to be but little rhodopsin in our 

 rods, for the concurrent processes of its synthesis and breakdown are 

 then in equilibrium at a sub-maximal concentration of the substance. 

 When we enter a dark place the process of adaptation to dim light 

 begins at once, since the breakdown all but ceases while the upbuilding 

 of new rhodopsin continues at the usual rate. In the dim light, a new 

 balance is struck at a high concentration of rhodopsin, so that a given 

 amount of additional light will now appear brighter than before, since it 

 destroys a greater absolute amount of the photosensitive pigment. 



Rhodopsin is not quite the whole story in dark-adaptation, however. 

 The dilation of the pupil, upon going into a dim or dark place, admits 

 more light to the retina, so that the overall sensitivity of the eye in- 

 creases somewhat, apart from any change in the retina itself. In the latter, 

 the first step in dark adaptation is taken by the cones rather than the 

 rods, for the tiny amount of photosensitive material which they ever 

 contain is very quickly built up to a maximum (see right half of Fig. 28) . 



