SCOTOPIA OR TWILIGHT VISION 



63 



identical with the luminosity curve of a feeble spectrum when the 

 maxima are made equal. Fig. 21 shows such a curve. 



Such are the conditions of the chromatic and general thresholds 

 in the dark-adapted eye. What are the conditions of extinction of 

 colour and light in the photopic eye ? This difficult problem has been 



150 

 140 



10 



021-68 10 12 14- 16 18 20 21 24 26 28 30 32 34 36 38 40 42 44% 48 50 52 54 56 55 60 62 64 66 

 Scale of Spectrum 



Fig. 20. '' Extinction of light " curve. Abscissae, wave-lengths of the prismatic spectrum ; 

 ordinates, the fractions of the beams which are just invisible, the unreduced illumina- 

 tion of the D light at the screen being one Hefner-foot (0-9 candle-foot). The dotted 

 lines show the proportion on the assumption that each beam has the same intensity 

 as that of the D light. 



attacked for the general threshold by Abney 1 in the following manner. 

 A disc of matt white paper, 4 inches in diameter, was illuminated by 

 white light. Through a hole in the centre, ,| inch in diameter, a coloured 

 light was viewed. The fovea was thus stimulated with monochromatic, 



1 Proc. Roy. Soc Lond. A, LXXXVII. 11)12, 



