32 LETTERS ON NATURAL MAGTC. 



and greens as blues, so that he is capable only of 

 appreciating blue and yellow colours. 



In all those cases which have been carefully 

 studied, at least in three of them, in which I have 

 had the advantage of making personal observa- 

 tions, namely, those of Mr. Troughton, Mr. 

 Dalton, and Mr. Liston, the eye is capable of 

 seeing the whole of the prismatic spectrum, the 

 red space appearing to be yellow. If the red 

 space consisted of homogeneous or simple red 

 rays, we should be led to infer that the eyes in 

 question were not insensible to red light, but 

 were merely incapable of discriminating between 

 the impressions of red and yellow light. I have 

 lately shown, however, that the prismatic spectrum 

 consists of three equal and coincident spectra of 

 red, yellow, and blue light, and consequently, that 

 much yellow and a small portion of blue light 

 exist in the red space ; and hence it follows, that 

 those eyes which see only two colours, viz. yellow 

 and blue, in the spectrum, are really insensible to 

 the red light of the spectrum, and see only the 

 yellow with the small portion of blue with which 

 the red is mixed. The faintness of the yellow 

 light which is thus seen in the red space, con- 

 firms the opinion that the retina has not ap- 

 preciated the influence of the simple red rays. 



If one of the two travellers who, in the fable 

 of the chameleon, are made to quarrel about the 

 colour of that singular animal, had happened to 

 possess this defect of sight, they would have 

 encountered at every step of their journey, new 

 grounds of dissension, without the chance Tof 

 finding an umpire who could pronounce a satis- 

 factory decision. Under certain circumstances, 



