228 



COLOUR VISION 



proportional to the areas of the three sensation curves. The relative 

 areas of these curves for the electric arc are RS 579, GS 248, BS 3-26. 



When considering the matches made between different lights it is 

 often convenient to adopt a different scale, viz., one in which the areas 

 of the three sensation curves are equal to each other. With this scale 

 equal ordinates of the three sensation curves correspond to a mixture 

 which will appear white to the normal eye. If we multiply the 

 ordinates of the green sensation curve by 579/248 = 2"21, and those of 

 the blue sensation curve by 579/3-26 = 117, the green and blue sensation 

 curves will have the same area as the red sensation curve. 



100 

 90 

 80 

 70 

 60 

 60 

 40 

 30 

 20 

 10 



64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 

 7o|oo" ' I lesjool I ' I 6o|oo • ' ' ' 5o[oo I I i I 5o[oo*u ' • I '" 



Fig. 63. Abney's E, G, and B sensation curves in terras of luminosity. The sums of 

 the ordinates at any point are equal to the ordinate of tlie photo pic luminosity 

 curve. (Watson.) 



In the curves in Fig. 63 the source of light was the crater of 

 the arc light with a horizontal positive pole. The source of light 

 in Abney's original investigations was the crater of the arc light with 

 sloping carbons, and the corresponding factors are 2-3 and 190. 



The luminosity curves are therefore brought to equal areas by 

 multiplying the GS and BS luminosity- values by 2-3 and 190 respectively, 

 thus giving the curves shown in Fig. 64, where equal stimulation of all 

 three components, i.e., equal ordinates, give the sensation of white. 

 Sir William Abney was the first to work out the colour sensations of 

 the normal trichromat exhaustively in this manner. 



