CHEMICAL BASIS OF THE ANIMAL BODY. 265 



(ii) Xanthophane. Soluble in ether, carbon bisulphide and in 

 alcohol. In ethereal solution it shows only one absorption band near 

 F towards the blue end of the spectrum. In carbon bisulphide it 

 shows similarly one band near, and to the blue side of b. It is thus 

 distinguished from the yellow pigment (lipochrin) of the retinal epi- 

 thelium previously described. 



(iii) Rhodophane. Soluble in turpentine, benzol and in alcohol. 

 In benzolic solution it shows one band close to, but on the red side of 

 F; in solution in turpentine the band is similarly near, but now on the 

 blue side of F. 



Solutions of the chromophanes are slowly bleached by the action of 

 light, chlorophane losing its colour fairly rapidly, xanthophane more 

 slowly and rhodophane only after prolonged exposure. In the less 

 pure form in which the chromophanes were first obtained by Kiihne 

 they gave the reactions which characterise the lipochromes or lutein, 

 viz. : (i) A transient violet followed by a bright blue when treated with 

 concentrated sulphuric acid, (ii) A transient bluish-green under the 

 influence of strong (yellow) nitric acid, (iii) An initial green colour 

 passing into bluish-green by the action of a dilute ('25 p. c.) solution 

 of iodine in dilute ('5 p. c.) iodide of potassium *. In the purer form in 

 which they were subsequently prepared, Kiihne found that they all 

 three gave the first of the above reactions, while none of them were 

 coloured by the iodine solution, and in the case of rhodophane the 

 second reaction with nitric acid was scarcely marked. 



4. Visual-purple (Rhodopsiri). 



This extremely unstable pigment may be stated to occur gene- 

 rally (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 2 . 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 

 immediate neighbourhood of the fovea centralis. It is entirely absent 

 from the cones, and hence is not found either in the fovea centralis of 

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



Preparation in solution. The most suitable material is afforded by 



1 See Capranica, Arch. f. Physiol. 1877, S. 283. For a conclusive reply to the 

 views as to the identity of these fatty pigments with lutein put forward in this paper 

 see Kiihne, Unters. a. d. physiol. Instil. Heidelb. Bd. iv. (1882), S. 169. 



2 The red colour of the retina of Cephalopods, first described by Krohn in 1839, 

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



