ON LIGHT. 259 



arising from their mixture cannot be separated by any 

 subsequent refraction into its components. 



(44.) In persons who are what is called " colour- 

 blind," the eye is sensible to all the rays of the prismatic 

 spectrum as light, though even in that respect the red 

 rays appear comparatively deficient in power to stimulate 

 the nerves of vision, so that all colours, into which a 

 large proportional admixture of primary red enters, are 

 described by them as sombre tints. But besides this, 

 two of the primary coloured rays, the red and the green, 

 appear to excite in their nerves sensations of colour 

 nearly or exactly similar. Their vision is therefore, in 

 fact, dichromic; all their compound colours are resolv- 

 able into two elements only instead of three. Red 

 they do not distinguish from green. The scarlet coat 

 of the soldier and the turf on which he is exercised 

 the ripe cherries and the green leaves among which 

 they hang are to them undistinguishable by colour, 

 though from constantly hearing them so spoken of, 

 they habitually speak of the fruit as red and the leaves 

 as green. Their sensation of blue is probably the same 

 as in normal vision ; though whether that excited by 

 their other colour, be such as a normal-eyed person 

 would call red, yellow, green, or something quite 

 different from either, we have no means of ascertain- 

 ing, nor can they give us any information. The face 

 of nature must appear, however, to them far inferior 

 in splendour and variety to that which we behold; 

 and if there be, as is asserted, here and there an in- 

 dividual totally destitute of the sensation of difference 



