ON THE THEORY OF COMPOUND COLOURS. 419 



with that for the composition of forces in mechanics. This analogy has been 

 well brought out by Professor Grassmann in Poggendorff's Annalen, Bd. LXXXIX. 



We may conceive an arrangement of actual colours in space founded upon 

 this construction. Suppose each of these radiating lines representing a given 

 colour to be itself illuminated with that colour, the brightness increasing from 

 zero at the origin to unity, where it cuts the plane of the diagram, and 

 becoming continually more intense in proportion to the distance from the origin. 

 In this way every colour in nature may be matched, both in quality and 

 quantity, by some point in this coloured space. 



If we take any three lines through the origin as axes, we may, by co-ordi- 

 nates parallel to these lines, express the position of any point in space. That 

 point will correspond to a colour which is the resultant of the three colours 

 represented by the three co-ordinates. 



This system of co-ordinates is an illustration of the resolution of a colour 

 into three components. According to the theory of Young, the human eye is 

 capable of three distinct primitive sensations of colour, which by their composition 

 in various proportions, produce the sensations of actual colour in all their varieties. 

 Whether any kinds of light have the power of exciting these primitive sensations 

 separately, has not yet been determined. 



If colours corresponding to the three primitive sensations can be exhibited, 

 then all colours, whether produced by light, disease, or imagination, are com- 

 pounded of these, and have their places within the triangle formed by joining 

 the three primaries. If the colours of the pure spectrum, as laid down on the 

 diagram, form a triangle, the colours at the angles may correspond to the primitive 

 sensations. If the curve of the spectrum does not reach the angles of the circum- 

 scribing triangle, then no colour in the spectrum, and therefore no colour in 

 nature, corresponds to any of the three primary sensations. 



The only data at present existing for determining the primary colours, are 

 derived from the comparison of observations of colour-equations by colour-blind, 

 and by normal eyes. The colour-blind equations differ from the others by the 

 non-existence of one of the elements of colour, the relation of which to known 

 colours can be ascertained. It appears, from observations made for me by two 

 colour-blind persons*, that the elementary sensation which they do not possess 

 is a red approaching to crimson, lying beyond both vermilion and carmine. These 



* Transactions of the Royal Society of Edinburgh, VoL XXL Pt 2, p. 286. 



532 



