PHYSIOLOGICAL 245 



into chemical energy being the most plausible "explanation" of 

 vision; but when we remember that the cones can discriminate 

 colours, we must go farther and assume that there are at least three 

 of these imagined substances, one for each of three primary colours ! 

 The sensitiveness of the retina is amazing. The amount of Ught- 

 energy required to evoke a response from it has been measured, and 

 found roughly equal to the amount of light-energy which would, 

 under ideal conditions, enter the eye in one second from a candle 

 burning five hundred miles away. The retina is two or three thousand 

 times as sensitive as any photographic plate or film. 



The retina is not perfectly uniform. In the centre, on the line of 

 the axis of the eye, is a small depression, the "yellow spot". Here, 

 the overlying layers of nerve-cells are thinned out; rods are absent, 

 and only cones are found — adaptations, as we may well suppose, for 

 increasing the sharpness of sight at the point where it is most 

 required. Further from the centre, towards the edges of the 

 sensitive area, of which we make less conscious use, the number of 

 rods relatively to the cones steadily increases. 



Armed with this conception, we can perhaps better understand the 

 familiar phenomena of "dazzling". As everyone knows, when one 

 comes from a dark room into bright light the sight is confused for 

 some moments, in spite of a powerful constriction of the pupils. 

 Very soon, however, the eye becomes "light-adapted", and vision 

 is clear again. Again, we know that on passing from the light to 

 the dark we are at first unable to see anything; but after a time the 

 eye becomes "dark-adapted" and objects are easily recognised. In 

 this case it seems that two processes play a part: first, the cone 

 mechanism has to be shunted out of action, and it takes a moment 

 or two for "after-images" in the cones to die away completely; 

 secondly, the rod mechanism has to be brought into action, and this 

 probably involves the formation of a supply of visual purple, which 

 takes time. The dark-adapted eye is excessively stimulated by 

 bright lights, which again means that some time must elapse before 

 the rod mechanism is shunted out of action. It may not be too 

 fanciful to regard the rod mechanism as a sort of "low gear" which 

 can be brought into use when conditions are too difiicult for the 

 cone mechanism to work successfully. But the cone mechanism 

 itself can be fatigued by prolonged exposure to too bright a light, 

 so that a dark cloud seems to obscure vision for some moments ; it 

 is interesting that if the bright light is coloured, other colours can 

 be clearly seen immediately with no sign of fatigue — a fact which 

 supports the view that there are in the retina different mechanisms 

 for several different primary colours. 



The question of colour-vision in the lower animals has been much 

 discussed and investigated by many different methods. Very careful 

 experiments have conclusively proved that honey-bees and probably 



