224 COLOUR VISION 



enclosed by the luminosity curve. Hence the area of the luminosity 

 curve represents the total brightness of the light which is formed into 

 the spectrum and is, therefore, a constant whatever the condition of 

 the vision of the person who makes the observation. This is at once 

 apparent if we remember that if the brightness of the whole recombined 

 spectrum is compared with the comparison white, since these whites 

 are derived from the same source and must, therefore, have exactly 

 the same composition, a setting which appears correct to one person 

 must also appear correct to any other, whatever the differences which 

 may exist between their vision. 



On the Young-Helmholtz theory the sensation produced by light 

 of any colour is the sum of the sensations due to stimulation of the 

 three component mechanisms. Konig and others obtained their curves 

 for the three components by matching light of one wave-length with a 

 mixture of lights of two other wave-lengths. Abney, using a similar 

 method, adopted the principle that the sum of the ordinates of the 

 luminosity-values of each component for any given wave-length is 

 equal to the ordinate of the luminosity curve of the spectrum for that 

 wave-length. He then proceeded thus^ : 



The red sensation can be perceived in purity at one end of the 

 spectrum. From the darkest red to a point near the C line, a little 

 above the red lithium line, the colour is the same, though, of course the 

 brightness varies, but the brighter red colour can be reduced so as to 

 form an exact match with the dark red, and no mixture of any colours 

 will give a red of the description we find at the end of the spectrum. 



At the violet end of the spectrum we also find that the colour is the 

 same throughout, from the extreme visible limit to a point not far 

 removed from G, but it is not for this reason to be accepted that the 

 colour is due to only one sensation. It might be due to two or three 

 sensations if they were stimulated in the same proportions along that 

 region, and if the identical colour could be produced by the combination 

 of other colours. Experiment shows that a combination of two colours 

 will under certain conditions make violet, and that instead of a simple 

 sensation of violet we have in this region a blue sensation combined 

 with a large proportion of red sensation. If we know the percentage 

 composition of the violet mixture we may provisionally use this part 

 of the spectrum as if it excited but one sensation, and subsequently 

 convert the results obtained with it into the true sensations. Thus 

 in calculating the percentage of red in any colour, that existing in 



1 Abney, Phil. Trans. Boy. Soc. Land, cxcin. 259, 1899 ; ccv. 333, 1905. 



