EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 149 



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., Part IL, 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 represent 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 circum- 

 ference. The position 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 shews 

 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 shewn 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 pro- 

 portions 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 



