136 Dr, Nicholl on a pecvliar Imperfection [Fe b . 



rays, they are acted upon by the red rays which are contained in 

 the mixed Hght to which they are also exposed, and the sensual 

 state A arises. 



When the retinae of these individuals are exposed to the action 

 of the fifth set of prismatic rays, the sensation seeing a colon?- 

 arises. This sensation is not the same as either of those which 

 arises from the action of the first, of the second, or of the third 

 set of prismatic rays. They describe it as not being the same as 

 seeing red, seeing orangey or seeing yellow. Wishing then to 

 distinguish by name the sensation which arises in the presence 

 of the fifth set of prismatic rays, they apply to it the name 

 which others use to denote the sensation produced in them by 

 the action of those rays ; they call it seeing hlne. These indi- 

 viduals confound dark blue with crimson. Crimson is a dark 

 shade of red. What they call seeing blue is then seeing a deep 

 dark shade of that colonr which is excited by the red ray. The 

 subjects of the two cases which 1 have recorded call the crimson 

 curtains of their respective beds blue by day-light, and red by 

 candle-light. That is, by day-light, they have the sensation 

 seeing a dark shade of red; whereas, by candle-light, they have 

 a fainter degree of that sensation, or the sensation seeing a 

 lighter shade of that colour. As they use the term blue to 

 denote seeing a dark shade of red, so do they use the term light 

 blue to denote seeing light shades of red , which other people call 

 pink, or they indiscrinnnately use the terms light blue and pink 

 to denote the same sensation. When then the darker shades of 

 blue are presented to the retinae of these individuals, as their 

 retinae are (as I have endeavoured to prove) insensible to blue 

 rays, they will not have the sensation seeing blue, but they will 

 be acted upon by red rays of a dark shade, which excite the 

 sensation seeing dark red. For if they did not see any colour 

 during the presence of dark blue rays, they would see black, and 

 seeing only a few red rays mixed with no colour, they, in fact, 

 see a dark shade of red. Mr. Buchanan informs me that he can- 

 not distinguish dark blue from black by candle-light, and he 

 says, that what gives him by day the sensation seeing dusky red 

 affects him so little by candle-light, that he then scarcely set's 

 any colour. These persons confound grass green with scarlet, 

 light blue with pink, dark blue with crimson, very dark red with 

 black. The fact then appears to be, that these individuals see 

 redj see orange, and see yellow, from the actions of the first> 

 second, and third sets of rays respectively ; but that, as they 

 advance in the prismatic range, their retinae are affected only by 

 the red rays of the mixed light to which they are also exposed, 

 they see only shades of red, varying as they advance towards the 

 most refrangible rays, from a sensation similar to that which is 

 excited by the first set of the prismatic rays to less vivid degrees 

 of the same sensation, until at length no sensual state being 



