405 



by placing at the points corresponding to these colours, weights 

 proportional to their intensities ; then the resultant colour will be 

 found at the centre of gravity, and its intensity will be the sum of 

 the intensities of the components. 



From the mathematical development of the theory of Newton's 

 diagram, it appears that if the positions of any three colours be 

 assumed on the diagram, and certain intensities of these adopted as 

 units, then the position of every other colour may be laid down from 

 its observed relation to these three. Hence Newton's assumption 

 that the colours of the spectrum are disposed in a certain manner in 

 the circumference of a circle, unless confirmed by experiment, must 

 be regarded as merely a rough conjecture, intended as an illustration 

 of his method, but not asserted as mathematically exact. From the 

 results of the present investigation, it appears that the colours of the 

 spectrum, as laid down according to Newton's method from actual 

 observation, lie, not in the circumference of a circle, but in the 

 periphery of a triangle, showing that all the colours of the spectrum 

 may be chromatically represented by three, which form the angles of 

 this triangle. 



Wave-length in millionths of Paris inch. 



Scarlet 2328, about one-third from line C to D., 



Green 1914, about one-quarter from E to F., 



Blue 1717, about half-way from F to G. 



The theory of three primary colours has been often proposed as 

 an interpretation of the phenomena of compound colours, but the 

 relation of these colours to the colours of the spectrum dees not 

 seem to have been distinctly understood till Dr. Young (Lectures on 

 Natural Philosophy, Kelland's edition, p. 345) enunciated his theory 

 of three primary sensations of colour which are excited in diiferent 

 proportions when different kinds of light enter the organ of vision. 

 According to this theory, the threefold character of colour, as perceived 

 by us, is due, not to a threefold composition of light, but to the 

 constitution of the visual apparatus which renders it capable of being 

 affected in three different ways, the relative amount of each sensation 

 being determined by the nature of the incident light. If we could 

 exhibit three colours corresponding to the three primary sensations, 

 each colour exciting one and one only of these sensations, then since 

 all other colours whatever must excite more than one primary sensa- 



