1822.] of Vision xcith Regard to Colours. 137 



produced in the retinae, they have a sensation similar to that 

 which is present in the absence of all luminous rays ; that is, 

 they see black. They have sensations of seeing colour excited 

 only by red and yellow rays, and by the various compounds of 

 these two rays ; and they have distinct sensations produced only 

 by strong rays of these sets of prismatic rays ; for all dark 

 shades of red, of orange, of brown, as well as deep green and 

 purple, are by them confounded with black, i. e. with the absence 

 of all colour. The subject of one of my cases saw but three 

 colours when he looked through a prism. Mr. Buchanan says 

 that the rainbow appears to him yellow in the centre, and blue 

 at the edges, which latter assertion proves that he uses the 

 terms red and blue to denote similar sensations. The order of 

 the prismatic rays being as follows : Red, orange, yellow, green^ 

 light blue, dark blue, violet. The corresponding order of sensa- 

 tions which arise in these individuals, in the presence of these 

 successive prismatic rays, are as follows : Red, orange, yellow^ 

 red, pink, dark red, dark indistinct colour, or black. 



The facts observable in the cases of these individuals seem to 

 agree with the conclusion which I arrived at in the earlier part 

 of this pamper ; namely, that the retinse of these individuals are 

 sensible to red and to yellow rays, and to the rays compounded 

 of these two sets of rays, or to orange rays, but that they are 

 insensible to green, to blue, and to violet rays. 



Some persons possess auditory organs, which enable them ta 

 discriminate a great variety of sounds, while other individuals, 

 who possess great quickness of hearing with regard to sound in. 

 general, are, nevertheless, unable to distinguish any great variety 

 in the kind of sounds which they hear. There is general sensibi- 

 lity of the auditory nerve required for the production of the 

 general sensation hearing ; there is a partial sensibility of that 

 nerve required for the production of varieties in the kind of that 

 sensation. If an auditory nerve be so formed as to be capable 

 of assuming a great variety of sensual states according to the 

 nature of the impressions made upon it, the possessor of that 

 nerve is said to possess a nice ear, while he whose auditory nerve 

 possesses only general sensibility is said to have an unmusical 

 ear. It is the same with regard to the retina. It may possess 

 general sensibility, enabling its possessor to see shape and extent 

 of surface accurately, while it is incapable of assuming the usual 

 variety of sensual states, thereby preventing the possessor of it 

 from seeing that variety with regard to colour, which the posses- 

 sors of the ordinary kinds of retinae are enabled to distinguish. 



