VISION 1007 



substance through which the retinal elements are excited by 

 luminous stimuli, it seems to fulfil an important function in adapt- 

 ing the retina i.e., rendering it more sensitive for vision in dim 

 light. In any case, its discovery is in itself so interesting and so 

 suggestive as a basis for future work, that a short account of the 

 properties of the substance cannot be omitted here. 



Visual Purple. If the eye of a frog or rabbit, which has been kept 

 in the dark, be cut out in a dimly-lighted chamber or in a chamber 

 illuminated only by red light, and the retina removed, it is seen, when 

 viewed in ordinary light, to be of a beautiful red or purple colour. 

 Exposed to bright light, the colour soon fades, passing through red and 

 orange to yellow, and then disappearing altogether. The yellow colour 

 is due to the formation of another pigment, visual yellow; the preceding 

 stages are due to the intermixture of this visual yellow with the un- 

 changed visual purple in different proportions. With the microscope 

 it may be seen that the pigment is entirely confined to the outer segment 

 of the rods, where it exists in most vertebrate animals. It may be ex- 

 tracted by a watery solution of bile-salts, and the properties of the 

 pigment in solution are very much the same 

 as its properties in situ ; light bleaches the 

 solution as it does the retina. Examined with 

 the spectroscope, the solution shows no definite 

 bands, but only a general absorption, which is 

 very slight in the red, and reaches its maxi- 

 mum in the yellowish-green. In accordance 

 with this, it is found that of all kinds of mono- 

 chromatic 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 Fig . 426 ._o p togram. Part 

 portion is bleached. And when the image of of retina of rabbit> the 

 an object possessing well-marked contrasts of eye O f which had been 

 light and shadow (e.g., a glass plate with strips directed to an illumin- 

 of black paper pasted on it at intervals, or a ated plate of glass cov- 

 window with dark bars) is allowed to fall on an ered with strips of black 

 eye otherwise protected from light, the pattern paper, 

 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 a substance, for it is 

 absent from the cones of all animals and the rods of some. Frogs 

 and rabbits can undoubtedly see at a time when, by continued 

 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 



