116 LETtERS ON NATURAL MAGIC, 



Mr. Troughton, Mr. Dalton, and Mr. Listen, 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, confirms the 

 opinion that the retina has not appreciated 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 

 of finding an umpire who could pronounce a satisfactory 

 decision. Under certain circumstances, indeed, the arbiter 

 might set aside the opinions of both the disputants, and 

 render it necessary to appeal to some higher authority 



to beg he'd tell 'cm if lie knew 



Whether the thing was red or blue. 



In the course of writing the preceding observations, an 

 ocular illusion occurred to myself of so extraordinary a 

 nature, that I am convinced it never was seen before, and 

 1 think it far from probable that it will ever bo s-vn 

 again. Upon directing iny eyes to the candles that were 



