294 MR J. CLERK MAXWELL ON COLOUR, 



Note III. 

 On the Theory of Compound Colours. 



Newton's theorem on the mixture of colours is to be found in his Opticks, 

 Book I., Pt. II., Prop. VI. 



In a 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 hue 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 Newton. 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 1 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 Newton's construction is by an examina- 



