264 VISUAL-PURPLE. 



4. Visual-purple (Rhodopsin). 



This extremely unstable pigment may be stated to occur gen- 

 erally (some few exceptions have been observed) in the retinae of 

 all vertebrates. It does not appear as yet to have been found 

 in the eye of invertebrates. 1 It is confined entirely to the outer 

 limbs of the rods, but while occurring in the majority of the rods 

 it is not found in all of them ; thus, it is absent in those situated 

 in the immediate neighbourhood of the ora serrata, and (in man 

 at least) it is wanting in the scantily disposed rods in the imme- 

 diate neighbourhood of the fovea centralis. It is entirely absent 

 from the cones, and hence is not found either in the fovea cen- 

 tralis of the human retina, or in the rod-free retina of reptiles. 



Preparation in solution. The most suitable material is afforded 

 by the retinae of frogs which have been kept in the dark for two 

 or three hours ; since in these animals not only is the visual-pur- 

 ple very marked and somewhat persistent under the action of light, 

 but further, the retina can be separated from the adjacent epithelium 

 with great ease and is free from blood. The necessary operation 

 for the removal of the retinse, as also all subsequent manipula- 

 tions, must be carried on in a feeble light from a sodium flame 

 to avoid bleaching. The retinas (20 30 suffice) are then extracted 

 for an hour in the dark with about 1 c.c. of a freshly prepared 

 2 5 p. c. solution of bile salts from ox-bile, which is finally fil- 

 tered. If brought into daylight and examined, the solution is 

 seen to possess a brilliant pinkish-purple colour, which rapidly 

 becomes red, yellow, and finally colourless, under the action of 

 light. A similar initial colour is observed in the retina in situ, 

 followed by the same change of colour when exposed to light, 

 the yellow being regarded as due to a ' visual-yellow ' (xanthopsin) 

 and perhaps the final colourless stage, since it admits of regenera- 

 tion in the dark into visual-purple if the retina is fresh and in 

 contact with its epithelium (see 773), may be spoken of as a 

 ' visual-white ' (leukopsin). 



Spectroscopic properties. Neither visual-purple nor visual-yel- 

 low gives any distinct absorption band ; there is a general absorp- 

 tion of the central parts of the spectrum easily seen between E and 

 G in the case of visual-purple, which changes into a general absorp- 

 tion of the violet end of the spectrum from F onwards as the 

 purple changes into yellow and finally disappears altogether. 



Action of light. White light, as also that from an electric 

 lamp or magnesium flame, bleaches visual-purple with extreme 

 rapidity, dependency upon the intensity of the illumination: 

 direct sunlight destroys the colour almost instantaneously. 



is dne 



1 The red colour of the retina of Cephalopods, first described by Krohn ill 1 839,, 

 lue to other pigments which are very resistent to the action of light. 



