464 VISUAL PURPLE OR RHODOPSIN. [BOOK I. 



With regard to the distribution of the visual purple in the animal 

 kingdom, it is to be remarked that whilst the rod-bearing retinae 

 of vertebrates generally possess it, in a few isolated animals it is 

 inexplicably absent. Thus a species of bat (Rhinolophus hipposideros) 

 has no purple, and hens and pigeons want it, though bats have none 

 but rods in their retinae, while the birds mentioned, with a prepon- 

 derance of cones, yet possess rods also. With these exceptions, all 

 vertebrates with rod-bearing retinae possess the visual purple, and 

 all invertebrates hitherto examined lack it. It is found in day- 

 loving and night-loving animals, in the sunward-flying eagle and 

 the nocturnal owl, in fishes which inhabit the sombre depths of the 

 ocean, and in the embryo into whose eye light has never fallen. 



Method of Kiihne's study of the visual purple and of the 



separation of changes which it undergoes by the action of light were 

 Visual Purple much aided by the discovery of the fact that the 

 or Rhodop- colouring matter is soluble in aqueous solutions contain- 

 ing from 2 5 p. c. of crystallized bile. 



Colourless crystallized bile is obtained by evaporating ox bile to dryness 

 on the water-bath after mixing it thoroughly with much animal charcoal. 

 The perfectly dry residue is "heated with absolute alcohol and a large excess 

 of ether is added to the filtered solution ; by this means the salts of the bile 

 acids are precipitated and ultimately acquire a crystalline structure. The 

 precipitate which consists of sodium glycocholate and taurocholate is termed 

 ' crystallized bile.' 



The perfect isolation of rhodopsin by this solvent is beset with 

 difficulties, the greatest of which is to avoid contamination with blood- 

 colouring matter. The retinae of certain animals disappoint all 

 attempts to free them from haemoglobin and are therefore unfit for 

 the extraction of visual purple. Fortunately the frog is not among 

 these. Twenty to thirty frog retinae separated in the chamber by 

 the aid of sodium light, are moistened with about 1 c.c. of a 2 p. c. 

 solution of bile salts and shaken, but without violence, for an hour. 

 The mixture is allowed to stand so as to allow of the subsidence of the 

 grosser particles, and the supernatant fluid afterwards poured on to a 

 filter. The solution thus obtained is of a red-purple colour, bleach- 

 ing to a water-like fluid on exposure to light. The solution is 

 perfectly clear and transparent and does not fluoresce or seem 

 opalescent, if absolutely free from fuscin. It may be concentrated 

 rapidly in vacuo, yielding solutions of progressively deeper tints of 

 purple and finally a dark residue resembling ammoniacal carmine, 

 containing dark violet or black amorphous particles. This mass 

 reacts to light after the manner of solutions. It is hygroscopic and 

 its amorphous particles redissolve. If the bile solution of rhodopsin 

 is thrown upon a dialyser the bile escapes, leaving a violet magma 

 capable of being bleached in the sunlight. 



