296 ME J. CLERK MAXWELL ON COLOUR, 



unknown to us. The determination of the exact nature of the pure sensations, 

 or of their relation to ordinary colours, is therefore impossible, unless we can pre- 

 vent them from interfering with each other as they do. It may be possible to ex- 

 perience sensations more pure than those directly produced by the spectrum, by 

 first exhausting the sensibility to one colour by protracted gazing, and then sud- 

 denly turning to its opposite. But if, as I suspect, colour-blindness be due to the 

 absence of one of these sensations, then the point D in diagram (2), which indi- 

 cates their absent sensation, indicates also our pure sensation, which we may 

 call red, but which we can never experience, because all kinds of light excite the 

 other sensations. 



Newton has stated one objection to his theory, as follows : — " Also, if only 

 two of the primary colours, which in the circle are opposite to one another, be mixed 

 in an equal proportion, the point Z" (the resultant tint) " shall fall upon the centre 

 " (neutral tint) ; " and yet the colour compounded of these two shall not be per- 

 fectly white, but some faint anonymous colour. For I could never yet, by mixing 

 only two primary colours, produce a perfect white.'''' This is confirmed by the ex- 

 periments of Helmholtz ; who, however has succeeded better with some pairs of 

 colours than with others. 



In my experiments on the spectrum, I came to the same result ; but it ap- 

 peared to me that the very peculiar appearance of the neutral tints produced was 

 owing to some optical effect taking place in the transparent part of the eye on 

 the mixture of two rays of very different refrangibility. Most eyes are by no 

 means achromatic, so that the images of objects illuminated with mixed light of 

 this kind appear divided into two different colours ; and even when there is no 

 distinct object, the mixtures become in some degree analysed, so as to present a 

 very strange, and certainly " anonymous " appearance. 



Additional Note on the more recent experiments of M. Helmholtz.* 



In his former memoir on the Theory of Compound Colours, f M. Helmholtz 

 arrived at the conclusion that only one pair of homogeneous colours, orange-yel- 

 low and indigo-blue, were strictly complementary. This result was shown by 

 Professor Grassmann:}: to be at variance with Newton's theory of compound 

 colours ; and although the reasoning was founded on intuitive rather than expe- 

 rimental truths, it pointed out the tests by which Newton's theory must be verified 

 or overthrown. In applying these tests, M. Helmholtz made use of an appara- 

 tus similar to that'described by M. Foucault, § by which a screen of white paper 

 is illuminated by the mixed light. The field of mixed colour is much larger than 



* Poggendorff's Annalen, Bd. xciv. (I am indebted for the perusal of this Memoir to Professor 

 Stokes.) 



f lb. Bd. lxxxvii. Annals of Philosophy, 1852, Part II. 



+ lb. Bd. lxxxix., Ann. Phil., 1854, April. 



§ lb. Bd. lxxxviii. Moigno, Cosmos, 1853, Tom. ii., p. 232. 



