Prof. J. D. Forbes on the Classification of Colours. 163 



For he slates (Prop. VI. p. 136, edit. 1730) expressly, "I 

 could never yet by mixing only two primary colours produce 

 a perfect white. Whetlier it may be compounded of a mixture 

 of three taken at equal distances in the circumference [of the 

 figure of a circular spectrum which he is describing] I do not 

 know; but of four or five I do not much question but it may. 

 But these are curiosities of little or no moment to the understand- 

 ing of the phcenomcna of Jiatiire. For in all whites produced 

 by nature there uses to be a mixture of all sorts of rays and 

 by consequence a composition of all colours'!'-." 



Every optician knows Newton's empirical rule for the esti- 

 mation of the colour produced by the mixture of any number 

 of the elementary colours of the spectrum and in any propor- 

 tions. But it is necessary here to repeat it, because it would 

 appear to simplify the scale of colours much indeed, were 

 colours only such as the composition of the coloured rays of 

 light present. Imagine a circle drawn whose centre is pure 

 white, and its circumference presents in order all the colours 

 of the spectrum in succession, and occupying arcs proportioned 

 to their lengths in the true spectrum, the two ends of the spec- 

 trum or the extreme violet and red coalescing at one placef. 

 Let these colours pass gradual!}', by the mixture of white light, 

 from their intensest development at the circumference to per- 

 fect whiteness at the centre of the circle. Suppose it were 

 required to find the effect of mixing two parts of green, one 

 of red and one of violet. At the circumference of the circle and 

 at the centre of the green space, describe a small circle whose 

 area is 2; at the middle points of the red and violet describe 

 circles whose areas are I ; find the centre of gravity of the 

 three circles : it will be found on the diagram over the tint 

 which the mixture of rays, if actually formed, will present. 



Since the quantity of colour" in any mixture is to be regarded 

 in this arbitrary construction as applied at the centre of the 

 arc appropriated to each colour, and since no two such points 

 are directly opposed to one another in the circle, it follows by 

 construction (as M. Biot has remarked^) that a perfect white 



* Optics, Book I. part 2, prop. iv. 



t The limits of the several colours will occur, according to the New- 

 tonian proportions, at the following angles in a continued graduation : — 



o / 



Red begins at 0*0 



Orange 60-46 



Yellow 94-57 



Green 149-38 



Blue 210-23 



Indigo 265-4 



Violet 299-15 



X Traite de Physique, vol. iii. p. 449. 'i'" '• J«> "i* ■' 



M 2 



