64 



COLOUR VISION 



and the parafoveal zone with white light, of known intensities. A small 

 spot of shadow was thrown upon the white disc from which the para- 

 foveal zone was illuminated. The blackness of this shadow is the same, 

 irrespective of the intensity of the white light, though it appears to be 

 different according to that intensity. The monochromatic light was 

 then reduced until the spot illuminated by it matched the shadow spot 

 in blackness. 



6 8 10 I? I* 13 10 50 n 24 26 28 30 32 34 36 38 40 43 4" % IS 50 52 54 56 58 60 62 64 66 



Fig. 21. " Persistency curve " of a deuteranope. The values are the reciprocals of the 

 " extinction of light " values, which are also shown. The " normal luminosity curve " 

 is the curve for higher intensities of light (the photopic luminosity curve). The 

 persistency curve is the achromatic scotopic luminosity curve (compare Fig. 14). 

 (Abney and Festing.) 



The illumination of the white area was modified by the annulus so 

 that intensities from 0'2 to 0'00078 foot-candle were obtained. Fig. 22 

 shows the logarithms of the intensities of the light for different wave- 

 lengths for extinction with different strengths of illumination of the 

 parafoveal zone with white light. 



Fig. 23 shows the corresponding persistency curves. These, as 

 before pointed out, give the luminosity curves of the spectrum at the 

 points of extinction, and we see that they correspond with Konig's 



