296 MR 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 cach 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 
tivo of the primary colours, which in the circle are opposite to one another, be mixed 
im an equal proportion, the point Z” (the resultant tint) “ shall fall upon the centre 
O” (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 Hetmuourz ; 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,} M. Hetmuoirz 
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 NEwrTon’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 Nrwton’s theory must be verified 
or overthrown. In applying these tests, M. HrLMHoiTz made use of an appara- 
tus similar to that{described by M. Foucauut,§ by which a screen of white paper 
is illuminated by the mixed light. The field of mixed colour is much larger than 
* Pocernporrr’s Annalen, Bd. xciy. (I am indebted for the perusal of this Memoir to Professor 
Stokes.) 
+ Ib. Bd. Ixxxvii. Annals of Philosophy, 1852, Part II. 
t Ib. Bd, Ixxxix., Ann. Phil., 1854, April. 
§ Ib. Bd. Ixxxviii. Moreno, Cosmos, 1853, Tom. ii., p. 232. 
