78 



COLOUR VISION 



If a red and a green light are mixed to match the D light (589*2 jm/x) 

 in hue and luminosity the D field is considerably larger than that of 

 the mixture, as are also the fields of the red and green separately. 



Colours of pigments do not give the same fields as the spectrum 

 colours with which they approximately match, since they are impure 

 colours (Abney^). 



Fig. 31 gives the data for the fields for the whole range of the 

 spectrum with three difl^erent intensities, the luminosities of the D line 

 being 3 "95, 0'99, and 0"45 foot-candle respectively. (In the first two 

 the aperture was 0*525 inch at 1 ft., in the last 0'086 inch at 1 ft.) 

 The abscissae are scale numbers (wave-lengths), the ordinates degrees 

 of field. 



Fig. 31. The temporal and nasal limits of the fields of vision for spectral colours at three 

 different intensities of illumination. (Abney.) 



It will be seen that when the temporal field reads 40° the nasal 

 reads 30°, and that as the field increases 7|° on the temporal side it 

 increases nearly 6° on the nasal, irrespective of the particular colour. 



The curves for variations in intensity of the light are particularly 

 interesting. They were taken in the horizontal directions only 

 (Fig. 32). 



The rays used were blue 430*3 /x/it, yellow 589*2 /x^, red 670*5 jjl/jl, green 

 SSN 41*7 (about 530 fxfM). Unit intensity was yellow = 3*95 foot-candles, 

 red = 0*45 ft.-c, green = 2*8 ft.-c, blue = 0*27 ft.-c. The abscissae 

 are intensities of light, the ordinates degrees of field. 



1 Abney. p. 203. 



