90 Mr Dal ton on the Vision of Colours. 



candle-light, however, it was astonishingly changed, not having 

 then any blue in it, but being what I called red, a colour 

 which forms a striking contrast to blue. Not then doubting 

 but that the change of colour would be equal to all, I requested 

 some of my friends to observe the phenomenon ; when I was 

 surprised to find they all agreed, that the colour was not ma- 

 terially different, from what it was by day-light, except my 

 brother, who saw it in the same light as myself. This observa- 

 tion clearly proved, that my vision was not like that of other 

 persons ; and, at the same time, that the difference between 

 day-light and candle-light, on some colours, was indefinitely 

 more perceptihle to me than to others. It was nearly two years 

 after that time, when I entei'ed upon an investigation of the 

 subject, having procured the assistance of a friend, who, to his 

 acquaintance with the theory of colours, joins a practical know- 

 ledge of their names and constitutions. I shall now proceed to 

 state the facts ascertained under the three following heads : 



I. An account of my own vision. 



II. An account of others whose vision has been found simi- 

 lar to mine. 



III. Observations on the probable cause of our anomalous 



vision. 



I. Of my own vision. — It may be proper to observe, that I 

 am short-sighted. Concave glasses of about five inches focus 

 suit me best. I can see distinctly at a proper distance ; and 

 am seldom hurt by too much or too little light ; nor yet with 

 long application. 



My observations began with the solar spectrum, or colour- 

 ed image of the sun, exhibited in a dark room by means of a 

 glass prism. I found that persons in general distinguish six 

 kinds of colour in the solar image ; namely, red, orange, yel- 

 low, green, blue, and purple. Newton, indeed, divides the pur- 

 ple into indigo and violet ; but the difference between him and 

 others is merely nominal. To me it is quite otherwise : — I 

 see only two or at most three distinctions. These I should call 

 yellow and blue ; or yellow, blue, and purple. My yellow com- 

 prehends the red, orange, yclloto, and green of others ; and 

 noy blue and purple coincide with theirs. That part of the 

 image which others call red, appears to me little more than a 



