Construction of a Colour Map. 499 



have A, A', and S, as before ; but $', the second spectrum 

 colour, does not occur, inasmuch as there is no spectrum 

 colour in which green is less than both the red and the violet. 

 Hence the green, which is represented by A, can be defined 

 only by the addition of white to a spectrum colour ; and the 

 purple, which is represented by A', can be defined only by 

 the fact that when added to a spectrum colour they can form 

 white. 



To see how what precedes is represented in the Colour Map 

 (Plate VIII. fig. 4), take any line perpendicular to the spec- 

 trum-line, say the line in the orange for which the Colour 

 Index is 1*0, and compare this line with fig. 1. S is the 

 point on the spectrum-line, V is the point at which the " line 

 of no violet" is crossed, and G and R the points in which the 

 lines of no green and no red respectively are crossed, and S' 

 represents the complementary spectrum colour, which is 

 represented on the thick line at the point marked 1*0. This 

 thick line, along which the figures are marked, represents the 

 spectrum which is complementary to that from Red to Yellow, 

 and itself extends from Violet, of which the colour index is 

 zero, to Blue, of which the colour index is infinite. A similar 

 line gives the complementary spectrum of the part from Blue 

 to Violet, and itself extends from Red when the colour index 

 is infinite to Yellow when it is zero. The region on the right 

 outside all the lines gives all the colours to be obtained by adding 

 white to a spectrum colour ; and to ascertain the amount of 

 each sensation, we have only to measure horizontally to the 

 line giving the zero of that sensation. The region on the left 

 outside all the lines gives all the colours capable of making- 

 white with spectrum colours ; and here, again, to ascertain 

 the amount of each sensation we have only to measure hori- 

 zontally to the line giving the zero of that sensation. It will 

 thus be seen that the whole map is really constructed on one 

 single principle. It is obvious that if a series of colours are 

 obtained by some definite law, their positions on the map will 

 lie on some line straight or curved. 



It remains to consider the spaces enclosed within the lines. 

 On the right between the spectrum-line and the nearest sensa- 

 tion zero-lines lies a space which has a real meaning, as the 

 points in it represent colours in which the sensations have 

 certain positive ratios to one another ; but these ratios give a 

 more intense colouring than the spectrum colours themselves, 

 and therefore such points cannot represent any colours which 

 can be seen by a normal eye, because, as was known to Newton, 

 every mixture of colours is more diluted than the spectrum 

 colour which it most nearly resembles. This region may be 



