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tion, they must find their places in Newton's diagram within the 

 triangle of which the three primary colours are the angles. 



Hence if Young's theory is true, the complete diagram of all colour, 

 as perceived by the human eye, will have the form of a triangle. 



The colours corresponding to the pure rays of the spectrum must 

 all lie within this triangle, and all colours in nature, being mixtures 

 of these, must lie within the line formed by the spectrum. If 

 therefore any colours of the spectrum correspond to the three pure 

 primary sensations, they will be found at the angles of the triangle, 

 and all the other colours will lie within the triangle. 



The other colours of the spectrum, though excited by uncom- 

 pounded light, are compound colours ; because the light, though 

 simple, has the power of exciting two or more colour-sensations in 

 different proportions, as, for instance, a blue-green ray, though not 

 compounded of blue rays and green rays, produces a sensation com- 

 pounded of those of blue and green. 



The three colours found by experiment to form the three angles 

 of the triangle formed by the spectrum on Newton's diagram, may 

 correspond to the three primary sensations. 



A different geometrical representation of the relations of colour 

 may be thus described. Take any point not in the plane of Newton's 

 diagram, draw a line from this point as origin through the point 

 representing a given colour on the plane, and produce them so that 

 the length of the line may be to the part cut off by the plane as the 

 intensity of the given colour is to that of the corresponding point on 

 Newton's diagram. In this way any colour may be represented by 

 a line drawn from the origin whose direction indicates the quality of 

 the colour, and whose length depends upon its intensity. The 

 resultant of two colours is represented by the diagonal of the paral- 

 lelogram formed on the lines representing the colours (see Prof. 

 Grassmann in Phil. Mag. April 1854). 



Taking three lines drawn from the origin through the points of 

 the diagram corresponding to the three primaries as the axes of 

 coordinates, we may express any colour as the resultant of definite 

 quantities of each of the three primaries, and the three elements of 

 colour will then be represented by the three dimensions of space. 



The experiments, the results of which are now before the Society, 

 were undertaken in order to ascertain the exact relations of the 



