1314 VISUAL PURPLE. 



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 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 

 lio-ht from a sodium flame to avoid bleaching. The retina' 

 ( 21 >— 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 regenera- 

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

 contact with its epithelium (see § 576), 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 Gr 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 dis- 

 appears altogether. 



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

 lamp or magnesium flame, bleaches visual purple with extreme 

 rapidity, dependently 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 tempera- 

 tures the effect of light is less, increases with rise of tempera- 

 ture, 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 



