THE SENSES 935 



shows no definite bands, but only a general absorption, which is very 

 slight in the red, and reaches its maximum in the yellowish-green. 

 In accordance with this, it is found that of all kinds of monochro- 

 matic light the yellowish-green rays bleach the purple most rapidly, 

 the red rays most slowly. 



If a portion of the retina is kept dark while the rest is exposed to 

 light, only the latter portion is bleached. And when the image of an 

 object possessing well-marked contrasts of light and shadow (e.g., a 

 glass plate with strips of black paper pasted on it at intervals, or a 

 window with dark bars) is allowed to fall on an eye otherwise pro- 

 tected from light, the pattern of the object is picked out on the retina 

 in purple and white. A veritable photograph or ' optogram ' may 

 thus be formed even on the retina of a living rabbit ; and if the eye 

 be rapidly excised, the picture may be 

 ' fixed ' by a solution of alum, and thus 

 rendered permanent. 



These facts certainly suggest that 

 light falling on the retina may cause in 

 some sensitive substance or substances 

 chemical changes, the products of which 

 stimulate the endings of the optic nerve, 

 and set up the impulses that result in 

 visual sensations. 



The visual purple cannot itself be such FlQ 

 a substance, for it is absent from the Part of retina of rabbit> 

 cones of all animals and the rods of the eye of which had been 

 some. Frogs and rabbits can un- directed to an illuminated 

 doubtedly see at a time when, by con- 

 tinued exposure to bright sunlight, the 

 purple must have been completely bleached. And although the 

 alleged absence of the pigment in the eye of the bat might seem 

 to afford a ready explanation of the proverbial ' blindness ' of 

 that animal, such a hasty deduction would be at once corrected 

 by the fact that birds with as sharp vision as the pigeon are 

 equally devoid of visual purple, while in other nocturnal animals, 

 like the owl, it is plentifully found. The most probable hypo- 

 thesis of the function of the visual purple is indeed that which 

 attributes to it the property, in virtue of its capacity for regenera- 

 tion in the dark, of adapting the eye for night or twilight vision 

 in other words, of increasing the sensitiveness of the retina for faint 

 light, especially of the shorter wave-lengths. If this is the case, 

 it is precisely in nocturnal animals that we should expect to find 

 it in large amount ; and recently visual purple has been obtained 

 from more than one species of bat (Trendelenburg) . The fact 

 that central vision (p. 946) in which the rodless fovea is con- 

 cerned is but little, if at all, susceptible of dark-adaptation, 

 while peripheral vision shows a marked capacity of adaptation, 

 agrees well with this hypothesis. We shall see later that there 

 is some evidence that it is the mere perception of luminous im- 



