go Colours and their Relations. [January, 



by causing the rays from three points in the spectrum to 

 fall On one point of the retina, — the result being the im- 

 pression of pure white, undistinguishable from the white 

 resulting from the combination of all the spectral colours. 

 The proportions required to constitute this white vary with 

 the points on either side of the central green from which 

 the rays are taken. It is impossible by mixing pigments to 

 produce a similar result. If carmine, chrome-yellow, and 

 indigo be mixed in certain proportions, the resulting im- 

 picssion on the eye is that of blackness, not of whiteness. 

 It is possible, indeed, by whirling a disc, painted with dif- 

 ferent proportions of red, green, and blue, to produce a 

 ish-white, but not that pure white which may be ob- 

 tained by combining the rays of the spectrum. 



Prof. Maxwell extended his observations to the case of 

 colour-blind persons. The eyes :: those whom he examined 

 were dichromic, — that is, sensible of only, two impressions of 

 colour. The central green of the spectrum appeared to 

 them white, as did a considerable extent on either side of it. 

 Beyond that, on the less refrangible side, all appeared of one 

 colour, which they termed yellow of different degrees of in- 

 tensity, shading off into darkness towards the red extremity : 

 while on the more refrangible side all appeared likewise of 

 one colour, which they called blue of different degrees of 

 intensity, shading off into darkness at the violet end. The 

 space from the fixed line A to E appears yellow, reaching 

 its maximum between D and E, while the blue reaches its 

 maximum at about two-thirds from F towards G. The 

 mean green ray produces a fainter impression on the 

 punctum ccecum in such eyes than in those of more perfect 

 visual power. In the dichromic eye, rays taken from dif- 

 ferent points of the regions on opposite sides of the central 

 green, when combined in certain proportions, produce the 

 impression of whiteness without the aid of a third ray. 

 But no admixture of blue and yellow will to such eyes 

 appear green. Any combination of these two will appear 

 white, — either a yellowish-white if the yellow be in excess, 

 or a bluish-white if the blue predominate. 



These experiments throw great light on the nature of 

 complementary colours. They show that to perfect eyes, 

 when two colours are complementary, one or both of them 

 must be compound colours, and that only in dichromic eyes 

 can two pure colours be regarded as complementary. To 

 such eyes, yellow and blue being the only colours distin- 

 guishable, are always complementary to each other. When 

 a perfect eye. however, after dwelling for a long time on a 



