30 MEMOIR OF DR. DALTON, AND 



as has been remarked sounds badly, Sir John Hersehel having 

 changed it to Dichromic vision, believing that one of the 

 three colours is lost to the eye entirely. Such a vision there 

 seems to be, but this extent has not been observed in any 

 instances, by Dr. George Wilson, who thinks that there is 

 no colour quite lost, although the power of perceiving be 

 feeble, and he names it Chromato-Pseudopsis, or a false 

 vision of colours. This he has translated by Sir D. Brewster's 

 term, colour-blindness, which appears much too strong when 

 we consider that some colours are well seen, and others seen 

 in part. It seems, in fact, to be an imperfection in the power 

 of distinguishing colours, which may exist to any extent, either 

 very slightly, as is seen in e very-day life, where, for example, 

 among the many workpeople in a large mill, only a few 

 are found fit for arranging yarns with accuracy. A nice 

 perception of colour is there a*valuable gift, and is paid for 

 accordingly. Or it may occur decidedly defective, as with Dr. 

 Dalton and others. Dalton's brother had the same defect, 

 and one or two others in the neighbourhood of Eaglesfield, 

 of whom I have lately heard. It is probable that there are 

 many gradations, beginning with deficient colour sight and 

 ending in Dichromic, or perhaps Monochromic or Achromic 

 vision, or true colour blindness. Dr. Wilson well remarks, 

 that Daltonism, under which it has been known, is not a 

 proper name for the peculiarity, as it connects his name 

 with a defect. Indeed few eyes are found equal to Dalton's, 

 if we judge of them by their results. Dr. Wilson has made 

 the remarkable discovery that this defect may almost be 

 called common. 



Dalton remained without giving anything to the public 

 until 1799. In the College his order showed itself in the 

 careful list of students and their lessons, still remaining. 

 Possibly his duties occupied too much of his time to allow 

 of experiment, but he comes out so suddenly after that as 

 physikist and chemist, that his time must have been spent 



