THE THEORY OF COLOURS IN RELATION TO COLOUR-BLINDNESS. 



ascertained; and it is easy to see that when two compound colours are com- 

 bined, their centre of gravity is the position of the new colour. 



The idea of this geometrical method of investigating colours is to be found 

 in Newton's Opticks (Book I., Part 2, Prop. 6), but I am not aware that it has 

 been ever employed in practice, except in the reduction of the experiments 

 which I have just made. The accuracy of the method depends entirely on the 

 truth of the theory of three sensations, and therefore its success is a testimony 

 in favour of that theory. 



Every possible colour must be included within the triangle rgv. White 

 will be found at some point, u\ within the triangle. If lines be drawn through 

 w to any point, the colour at that point will vary in hue according to the 

 angular position of the line drawn to w, and the purity of the tint will depend 

 on the length of that line. 



Though the homogeneous rays of the prismatic spectrum are absolutely pure 

 in themselves, yet they do hot give rise to the "pure sensations" of which we 

 are speaking. Every ray of the- spectrum gives rise to all three sensations, 

 though in different proportions ; hence the position of the colours of the spectrum 

 is not at the boundary of the triangle, but in some curve C R Y G B V 

 considerably within the triangle. The nature of this curve is not yet determined, 

 but may form the subject of a future investigation *. 



All natural colours must be within this curve, and all ordinary pigments 

 do in fact lie very much within it. The experiments on the colours of the 

 spectrum which I have made are not brought to the same degree of accuracy as 

 those on coloured papers. I therefore proceed at once to describe the mode of 

 making those experiments which I have found most simple and convenient. 



The coloured paper is cut into the form of discs, each with a small hole 

 in the centre, and divided along a radius, so as to admit 

 of several of them being placed on the same axis, so that 

 part of each is exposed. By slipping one disc over another, 

 we can expose any given portion of each colour. These 

 discs are placed on a little top or teetotum, consisting of 

 a flat disc of tin-plate and a vertical axis of ivory. This 

 axis passes through the centre of the discs, and the quantity of each colour exposed 

 is measured by a graduation on the rim of the disc, which is divided into 100 parts. 



* [See the author's Memoir in the Philosophical Transactions, I860, on the Theory of Compound 

 Colours, and on the relations of the Colours of the Spectrum.] 



