152 EXPERIMENTS ON COLOUR, AB PERCEIVED BY THE EYE. 



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- 

 yellow and indigo-blue, were strictly complementary. This result was shewn by 

 Professor GrassmannJ to be at variance with Newton's theory of compound 

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

 t.-.\l>erimental 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 apparatus 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 in M. Helmholtz's former experiments, and the facility of 

 forming combinations is much increased. In this memoir the mathematical theory 

 of Newton's circle, and of the curve formed by the spectrum, with its possible 

 transformations, is completely stated, and the form of this curve is in some 

 degree indicated, as far as the determination of the colours which lie on oppo- 

 site sides of white, and of those which lie opposite the part of the curve which 

 is wanting. The colours between red and yellow-green are complementary to 

 colours between blue-green and violet, and those between yellow-green and blue- 

 green have no homogeneous complementaries, but must be neutralized by various 

 hues of purple, i.e., mixtures of red and violet. The names of the complementary 

 colours, with their wave-lengths in air, as deduced from Fraunhofer's measure- 

 ments, are given in the following table : 



* PoggendorfFa Annalen, Bd. XCIV. (I am indebted for the perusal of this Memoir to Professor 

 Stokes.) 



t 76. Bd. LXXXVII. Annals of Philosophy, 1852, Part n. 



* 76. Bd. LXXXIX. Ann. Phil., 1854, April. 



S Ib. Bd. LXXXVIII. Moigno, Cosmos, 1853, Tom. u., p. 232. 



