266 VISUAL PURPLE. 



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 purple 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 

 retinae, as also all subsequent manipulations, must be carried on in a 

 feeble light from a sodium flame to avoid bleaching. The retinae 

 (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 filtered. 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 regeneration 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 yellow 

 gives any distinct absorption band ; there is a general absorption of the 

 central parts of the spectrum easily seen between E and G in the case 

 of visual purple, which changes into a general absorption 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, de- 

 pendently upon the intensity of the illumination: direct sunlight 

 destroys the colour almost instantaneously. When monochromatic 

 light (of the spectrum) is used it is found that the yellowish-green, i.e. 

 the region most strongly absorbed by the pigment, is most active, 

 followed seriatim by green, blue, greenish-yellow, yellow, violet, orange 

 and red : the ultra-red rays have no such bleaching power. At low 

 temperatures the effect of light is less, increases with rise of tempe- 

 rature, and at 75 the colour is destroyed even without exposure to 

 light. 



Action of reagents. The colour is at once destroyed by the action 

 of caustic alkalis, most acids, alcohols, chloroform and ether : it is on 

 the other hand persistent in presence of ammonia, solutions of ordinary 

 alum, of sodium chloride, carbonates of the alkalis and a large number 

 of other salts. One of the most important factors in determining the 

 bleaching of visual purple by either light or heat is the presence or 

 absence of water. If the entire retina be dried in vacuo over sulphuric 



