122 



COLOUR VISION 



Following the matter out further we should eventually arrive at the 

 centre of an area as the part which was the recipient of the greatest 

 amount of the radiated stimuli, and consequently that would be the 

 last to disappear. With a slit aperture the slit is visible till extinction 

 is very nearly executed, but it finally merges into a fuzzy spot at the 

 moment before it finally fails to make any impression of light." 



Fig. 40 shows that there is an angular aperture or size of retinal 

 area stimulated at which any ray will be extinguished both for colour 

 and light at the same time. The aperture is largest for red. 



-i -a -J -4 -5 -6 



Diameter of apertures vn powers of 2. 



Fig. 40. Extinction of colour and light curves with different areas of stimulation. SSN 

 44 = 548- 1 fjifi ; SSN 50-6 =D line: I and II, different observers and different in- 

 tensities. (Abney.) 



Abney has also compared the luminosity of the spectral light from 

 apertures of different size. Fig. 41 shows the curves for scale numbers 



