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MR J. CLERK MAXWELL ON COLOUR, 
Nore III. 
On the Theory of Compound Colours. _ 
Newron’s theorem on the mixture of colours is to be found in his Opticks, 
Book L., Pt. II., Prop. VI. 
Ina mixture of primary colours, the quantity and quality of each being given, 
to know the colour of the compound. 
He divides the circumference of a circle into parts proportional to the seven 
musical intervals, in accordance with his opinion of the divisions of the spectrum. 
He then conceives the colours of the spectrum arranged round the circle, and at 
the centre of gravity of each of the seven arcs he places a little circle, the area of 
which represents the number of rays of the corresponding colour which enter into 
the given mixture. He takes the centre of gravity of all these circles to repre- 
sent the colour formed by the mixture. The /we is determined by drawing a line 
through the centre of the circle and this point to the circumference. The posi- 
tion of this line points out the colour of the spectrum which the mixture most 
resembles, and the distance of the resultant tint from the centre determines the 
fulness of its colour. 
Newton, by this construction (for which he gives no reasons), plainly shows 
that he considered it possible to find a place within his circle for every possible 
colour, and that the entire nature of any compound colour may be known from 
its place in the circle. It will be seen that the same colour may be compounded 
from the colours of the spectrum in an infinite variety of ways. The apparent 
identity of all these mixtures, which are optically different, as may be shown by 
the prism, implies some law of vision not explicitly stated by Newron. This 
law, if NewTon’s method be true, must be that which I have endeavoured to 
establish, namely, the threefold nature of sensible colour. 
With respect to NewTon’s construction, we now know that the proportions of 
the colours of the spectrum vary with the nature of the refracting medium. The 
only absolute index of the kind of light is the time of its vibration. The length of 
its vibration depends on the medium in which it is; and if any proportions are 
to be sought among the wave-lengths of the colours, they must be determined for 
those tissues of the eye in which their physical effects are supposed to terminate. 
It may be remarked, that the apparent colour of the spectrum changes most ra- 
pidly at three points, which lie respectively in the yellow, between blue and 
green, and between violet and blue. The wave-lengths of the corresponding rays 
in water are in the proportions of three geometric means between | and 2 very 
nearly. This result, however, is not to be considered established, unless con- 
firmed by better observations than mine. 
The only safe method of completing Newron’s construction is by an examina- 
