1822.] of Vision with Regard to Colours. 135 



denote the sensation arising from the action of the third set of 

 rays ; they call it seeing yellow. 



It will be recollected that when the sensation seeing red has 

 been for a long time kept up in ordinary cases by the continued 

 action of the first set of the prismatic rays, the sensation seeing 

 green arises, and vice versa. But (in the case of Mr. Buchanan*) 

 when the long continued action of the first set of prismatic rays 

 had produced the long continuance of seeing colour (which he 

 also terms seeing red), no other sensation of seeing a different 

 colour arose ; so that, in his case, the long continuance of seeing 

 red does not produce seeing green y as it does in ordinary cases. 



When the retinae of these individuals are exposed to the action, 

 of the fourth set of the prismatic rays, a sensation seeing a colour 

 is present, which they describe as being the same as that which 

 is produced by the action of the first set of prismatic rays. The 

 first and the fourth set of rays do not then produce in these per- 

 sons two different kinds of sensation. If then the first set of 

 the prismatic rays produces a peculiar corresponding sensation, 

 the fourth set of those rays does not produce such sensation as I 

 have already endeavoured to prove; so that if we admit that 

 these persons can have the sensation seeing red produced, we 

 must deny their being able to see green; we must suppose their 

 retinai to be insensible to the green rays ; to be, in other words, 

 incapable of assuming the sensual state D ; and this inference 

 seems confirmed by the fact of no new sensation arising in Mr. 

 Buchanan after the long continuance of that sensation seeing 

 colour which is produced in him by the long-continued action of 

 the red ray upon his retina. We are then, I think, warranted in 

 supposing that these individuals do not see green. Why then do 

 they see red in the presence of the green ray ? As far as regards 

 the green ray, they are in the state of a nyctalopic, whose retina 

 is sensible only to the red rays which are contained in mixed 

 light. They are in a state similar to that of persons whose 

 retinas have been long acted upon by green rays, and who then 

 have the sensation seeing red, although their retinae are still acted 

 upon by green rays. The retinae of these persons cannot assume 

 the sensual state D; the presence of the green ray, therefore, 

 does not produce any sensation. As then the sensation seeing 

 red arises in the presence of the green ray, and as such sensa- 

 tion can be produced only by the action of red rays, such 

 sensation must arise from the action of the most powerful of the 

 rays which are contained in the tnixed light to which their 

 retinsB are also exposed ; namely, of the red rays ; so that when 

 their retinae are exposed to green rays, since they cannot assume 

 the state D, and as they are sensible only to red, orange, or yellow 



• Medico-Chirurg. Trans, vol. ix. Part II. The experiment was not made in the 

 other case. 



