62 



COLOUR VISION 



reduction necessary when the illumination on the screen by each 

 monochromatic light was 1 foot-candle were calculated. These results 

 give the actual illumination at the point of extinction of colour in 

 terms of foot-candles. Thus, when the illumination with SSN'^ 50 

 {558[x[x) was 0"0016 foot-candle at the screen the colour just dis- 

 appeared, and so on. Fig. 19 shows one of the curves obtained. 



S (0 15 20 25 30 3S 40 'iS 50 SS 60 



Fig- 19. " Extinction of colour " curve. Abscissae, wave-lengths of the prismatic 

 spectrum ; ordinates, intensity of the illumination in candle-feet on the screen when 

 the colour just vanishes. (Abney.) 



In the " extinction of light " a curve was plotted showing what was 

 the fraction of the beam from each part of the spectrum which was just 

 invisible. The absolute luminosity of each part of the spectrum was 

 determined, and from these values a second curve was plotted with 

 ordinates representing the absolute luminosities at the points of extinc- 

 tion. The second curve shows the illumination in fractions of a foot- 

 candle at the screen by each monochromatic light which would be just 

 invisible. The branching of the curves (Fig. 20) beyond the green 

 towards the violet end is due to macular pigmentation, the " whole eye " 

 curve showing the intensities when the eye was allowed to wander. 

 If a curve is plotted from the reciprocals of the extinctions the " per- 

 sistency curve " (Abney and Festing) is obtained.. This is obviously 

 the luminosity curve of the spectrum at the points of extinction. It is 



1 SSN is the scale-number on Abney's arbitrary scale of the spectrum. 



