34 COLOUR VISION 



and of yellow of 580 ju/x causes a colour sensation which can be accurately 

 matched with that derived from some spectral colour between those 

 wave-lengths. The exact position, i.e. the wave-length, of this colour 

 depends upon the relative amounts of the two colours in the mixture. 

 If there is an excess of red the resultant mixture will match a colour 

 nearer the red than the yellow-green. Moreover, the position of the 

 colour will be accurately represented by the mass centre of the weights 

 of the two components, i.e. by the centre of gravity, as Newton showed.- 

 v. Kries 1 gives the following matches of mixtures of 670'8 ^^ and 

 552 f^/ji for the spectrum of gas light, the measurements being obtained 

 with the Helmholtz-Konig spectrophotometer : 



Amounts of red (070 '8 MM) and 



green-yellow (552 MM) in Spectral match 



the mixtures 



070 8 MM 552 MM MM 



1 00 t>70-8 



2-84 0-14 028 



3-11 0-38 615 



3-05 0-09 003 



2-27 0-94 51)1 



1-39 107 581 



0-82 1-28 571 



0-24 1-13 501 



1-00 552 



All the colour sensations in this region are therefore functions of a 

 single variable and can be represented on a straight line^ AB, where A 

 represents about 800 ^p, and B 540 up,. Each point on the line represents 



A C D B 



I 1 1 1 



Fig. 6. Representation of mixtures of lights of long wave-length on a straight line. 



a colour sensation. If an amount, m } , of light L\, at A is mixed with 

 an amount, w 2 , of light, L 2 , at B, then the resultant sensation is a light, 

 Ly, at C such that CA : CB as m.> : w, . Similarly, if m :t of L, at C be 

 mixed with w 4 of L., at B, the resultant sensation is a light, L 4 , at D, 

 such that DC : DB as m^ : m s , and so on. All mixtures therefore 

 which give a sensation corresponding to a given point give rise to the 

 same sensation or match accurately. Hence the range from 800/n/iA to 

 540 /u/u, must, like the purple range, be represented in the colour diagram 

 as a straight line. 



1 Zitsch. f. Psyckol. u. Physiol. d. iSinnesorg. xin. 281, 1890. 



