288 MR J. CLERK MAXWELL ON COLOUR, 
In one experiment, after looking at a bright light, with a red glass over one 
eye and a green over the other, the two tints in experiment (4) appeared to him 
altered, so that the outer circle was lighter according to one eye, and the inner 
according to the other. As far as I could ascertain, it appeared as if the eye 
which had used the red glass saw the red circle brightest. This result, which 
seems at variance with what might be expected, I have had no opportunity of 
verifying. 
This paper is already longer than was originally intended. For further infor- 
mation I would refer the reader to Newton’s Opticks, Book I. Part II., to Youna’s 
Lectures on Natural Philosophy, page 345, to Mr D. R. Hay’s works on Colours, 
and to Professor Forses on the Classification of Colours (Phil. Mag., March 1849). 
The most remarkable paper on the subject is that of M. Hetmuotrz, in the 
Philosophical Magazine for 1852, in which he discusses the different theories of 
primary colours, and describes his method of mixing the colours of the spectrum. 
An examination of the results of M. Hetmuoirz with reference to the theory 
of three elements of colour, by Professor GRASSMANN, is translated in the Phil. 
Mag., April 1854. 
References to authors on colour-blindness are given in Dr G. Winson’s papers 
on that subject. A valuable Letter of Sir J. F. W. Herscuex to Datron on his 
peculiarity of vision, is to be found in the Life of Datron by Dr Henry. 
I had intended to describe some experiments on the propriety of the method 
of mixing colours by rotation, which might serve as an extension of Mr Swan’s 
experiments on instantaneous impressions on the eye. These, together with the 
explanation of some phenomena which seem to be at variance with the theory of 
vision here adopted, must be deferred for the present. On some future occasion, 
I hope to be able to connect these simple experiments on the colours of pigments 
with others in which the pure hues of the spectrum are used. I have already 
constructed a model of apparatus for this purpose, and the results obtained are 
sufficiently remarkable to encourage perseverance. 
Nore I. 
On diferent Methods of Exhibiting the Mixtures of Colours. 
(1.) Mechanical Mixture of Coloured Powders. 
By grinding coloured powders together, the differently-coloured particles may 
be so intermingled that the eye cannot distinguish the colours of the separate 
powders, but receives the impression of a uniform tint, depending on the nature 
and proportions of the pigments used. In this way, NEwron mixed the powders 
of orpiment, purple, bise, and viride wris, so as to form a gray, which, in sunlight, 
resembled white paper in the shade. (Newron’s Opticks, Book I. Part II. Exp. 
