ON THE THEORY OF COMPOUND COLOURS. 431 



From these equations we may lay down a chart of the spectrum on Newton's 

 diagram by the following method : Take any three points, A, B, C, and let A 

 represent the standard colour (24), B (44), and C (68). Then, to find the position 

 of any other colour, say (20), divide AC in P so that (18 '6) AP=(2-8)PC, and 

 then divide BP in Q so that (l8-6 + 2'8)P# = (0-4) QB. At the point Q the 

 colour corresponding to (20) must be placed. In this way the diagram of fig. 4, 

 Plate VI., p. 444, has been constructed from the observations of all the colours. 



XL TJie Spectrum as laid down on Newton's Diagram. 



The curve on which these points lie has this striking feature, that two 

 portions of it are nearly, if not quite, straight lines. One of these portions 

 extends from (24) to (46), and the other from (48) to (64). The colour (20) 

 and those beyond (64), are not far from the line joining (24) and (68). The 

 spectrum, therefore, as exhibited in Newton's diagram, forms two sides of a 

 triangle, with doubtful fragments of the third side. Now if three colours in 

 Newton's diagram lie in a straight line, the middle one is a compound of the 

 two others. Hence all the colours of the spectrum may be compounded of 

 those which lie at the angles of this triangle. These correspond to the following 

 colours : 



TABLE VII. 



Index Wave-length 



in water. 



1-747 

 1-435 

 1-282 



All the other colours of the spectrum may be produced by combinations of 

 these; and since all natural colours are compounded of the colours of the spec- 

 trum, they may be compounded of these three primary colours. I have strong 

 reason to believe that these are the three primary colours corresponding to three 

 modes of sensation in the organ of vision, on which the whole system of colour, 

 as seen by the normal eye, depends. 



XII. Results found by a second Observer. 



We may now consider the results of three series of observations made by 

 myself (J.) as observer, in order to determine the relation of one observer to 



