SCOTOPIA OR TWILIGHT VISION 61 



threshold, the specific threshold remaining almost or quite constant. 

 As regards the nature of the light stimulus the colourless interval is 

 greatest for light of short wave-length and least for light of long wave- 

 length. In the orange it is small even with good dark adaptation. 

 In the red beyond 670 /^/z, it is almost completely abolished. In fact 

 even with very good dark adaptation such a red light excites the 

 sensation of red 1 , and the only evidence of a colourless interval is the 

 alteration in the character of the sensation as dark adaptation becomes 

 more complete, the red disappearing or becoming paler and brighter 

 according to whether its wave-length is greater or less than about 

 670/x//,. A minimal colourless interval can, however, be elicited under 

 suitable conditions degree of dark adaptation, size of field, paracentral 

 or peripheral stimulation (Charpentier) 2 (v. p. 81). 



The general light-threshold has been held to meet the requirements 

 of a physiological unit of luminosity, for it has been accepted as an 

 axiom that at their achromatic scotopic thresholds all lights are of equal 

 brightness. ; The light which can just be perceived has always the 

 same brightness, no matter what be the light which acts as the stimulus" 

 (Konig 3 ). 



Abney and Festing 4 early recognised the importance of investi- 

 gating what are here called the chromatic and general thresholds for 

 different parts of the spectrum. Light from a monochromatic beam 

 was matched with that from a white beam, both being altered in intensity 

 by the use of rotating sectors (episcotister), or in Abney's later experi- 

 ments by the annulus (p. 5). Accurate photometric observations of 

 the comparison light were made and the experimental variations were 

 calculated from the aperture of the sectors or the scale of the annulus. 

 The " extinction of colour " was measured thus. The light from the 

 D line was taken as a standard and was arranged so that the illumina- 

 tion on the screen was 1 foot-candle. The intensity of the monochromatic 

 light was then reduced by the annulus or episcotister until it appeared 

 colourless and matched the comparison '" white." The amounts of 

 reduction necessary for various monochromatic lights throughout the 

 spectrum were thus obtained. From these results the amounts of 



1 Parinaud, Compt. rend, dc VAcad. fr., 286. 1881 ; Konig, p. 144; v. Kries, Ztsck. f. 

 Paychol. u. Physiol. d. tiiiniesorg. ix. 86, 1896. 



2 Arch, de Physiol. 1877 ; Arch. d'Opht. xvi. 337, 1896 ; Bering, Arch. f. d. ges. Physiol. 

 LX. 535, 1895; Koster,Arch.f. Ophlh. XLI. 4, 13, 1895; Tschermak, Arch. f. d. ges. Physiol. 

 LXX. 320, 1898 ; v. Kries and Nagcl, Ztsch. f. Psychol. u. Physiol. d. Ximtcxorij. xn. 

 15, 1896. 



3 Konig, p. 190. 4 Phil. Trans. Hoy. Soc. Loud. CLXXXJII. 537, 1892. 



