

8i6 A MANUAL OF PHYSIOLOGY 



tive 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; 

 FIG. 314. OPTOGRAM. the preceding stages are due to the inter- 

 Pan of retina of rabbit, mixture of this U *} yellow with the un- 

 the eye of which had been changed visual purple m different proper- 

 directed to an illuminated tions. With the microscope it may be seen 

 With that the pigment is entirely confined to the 

 outer segment of the rods, where it exists in 

 most vertebrate animals. It may be extracted 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 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 end- 

 ings 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. 



