SCOTOPIA OR TWILIGHT VISION 



55 



will differ from the prismatic curve. Krarup^, using the energy values 

 for the Hefner light calculated by Angstrom, and allowing for absorption 

 by the macular pigment on the basis of Sachs's researches, has calculated 

 the achromatic scotopic luminosity curve from Konig's results. This 

 curve is independent of the source of light employed and shows a 

 remarkable agreement with (1) the curve of absorption of visual 

 purple ; (2) the luminosity curve of the totally colour-blind {vide 

 infra) ; and (3) the similarly corrected achromatic scotopic luminosity 

 curve of a protanope investigated by Konig. 



Sunlight 



Nernstlight 

 Gaslight 





^2 S 



Iff If III I 





Fig. 10. Scotopic luminosity curves for direct sunlight (Schaternikofi'), gas light (Schater- 

 nikoff), and Nernst light (Trendelenburg), reflected from a magnesium oxide-coated 

 surface. Abscissae, wave-lengths of the prismatic spectrum ; ordinates, arbitrary 

 scale, the maximum of luminosity being in each case 3000. (Trendelenburg.) 



The flicker method 



gives 



concordant results. Fig. 17 shows 



Haycraft's^ results for colour-discs with three intensities. The ordinates 

 show the number of revolutions of a semi-disc per second which were 

 found necessary to abolish flicker : the abscissae are wave-lengths. 



The extraordinary similarity between the scotopic luminosity curve 

 and the curve of values of different monochromatic lights for bleaching 

 frog's visual purple (Trendelenburg) (Fig. 1), seems to point to some 

 underlying physiological fact of great significance. We shall return to 

 this point later. 



^ Loc. cit. p. 21. 



2 J. of Physiol, XXI. 12(». 1897. 



