44 COLOUR VISION 



Whether or not we agree with these words, that a high degree of 

 accuracy in such estimations can be attained is shown by Abney's 

 results, notably the following 1 : 



" To test the illuminating value of colour mixtures, three slits were 

 placed in the spectrum, in the red, green and violet. The luminosities 

 of the rays coming through each were measured (1) separately ; 

 (2) in pairs ; (3) the whole combined. 



R 203 



(R + G) 242 



G 38-5 



(G+ V) 45 



V 8-5 



(R + V) 214 



(R+ G+ V) 250 



Combining these together w T e get : 



R + G+ 7=250 

 (R + G)+ F=250'5 

 (R + V)+ G =252-5 

 (0+ V)+ =248 

 (R+ G+ F)=250 



= 250'25 by least squares. 



Within the limits of error of observation the luminosity of the 

 combined spectrum measured as white is equal to the luminosity of 

 spectrum colours measured separately." 



The very fairly coordinated results of different observers and 

 different methods show that in these luminosity measurements there is 

 something measured which appeals to their senses as of the same order. 

 Most will agree that it is a complex " something " which is expressed 

 by v. Helmholtz as a combination of " brightness and colour glow," 

 and which we shall see later is an integral part of Hering's theory and 

 is expressed as the " white valency " of the colours. Further analysis 

 of this " something " must be left till a later stage. In the meantime 

 we will designate it " luminosity." 



Direct comparison or "equality of brightness 2 " is not the only method 

 of measuring the luminosity of spectral colours and mixtures. So 

 early as 1735 Celsius employed the visual acuity to determine the relative 

 brightness of various regions of the spectrum, and this method was also 

 employed by Buffon and Sir Wm. Herschel 3 . The same principle has 

 been applied to white light in the discrimination photometers of Houston 



1 Abney, p. 105. 2 This is the term usually applied to this method by physicists. 



3 Parsons, Roy. Land. Ophth. Hosp. Rep. xix. 277, 1914. 



