SCOTOPIA OR TWILIGHT VISION 53 



A spectrum of low intensity appears as a colourless bright streak, 

 varying, however, in brightness in different parts. Consequently 

 accurate matches can be made between any two parts of the spectrum 

 by merely modifying the intensity of one light. 



If the intensity of the spectrum is slightly raised the colours become 

 evident in a definite order and the relative brightness of the different 

 parts becomes altered. As the intensity is still further raised the eye 

 becomes light adapted and the spectrum shows all its hues with the 

 relative brightnesses described in Section II. Scotopic vision at very 

 low intensities is therefore achromatic ; with slightly raised intensities 

 of light it becomes chromatic. We may distinguish the two conditions 

 as achromatic and chromatic scotopia respectively. 



The achromatic scotopic values of different parts of the spectrum 

 were first investigated by Hering and Hillebrand^ for the dispersion 

 spectrum of daylight. Abney and Festing's curve^ is shown in Fig. 14. 



The striking feature is that the brightest part, instead of being in 

 the neighbourhood of the D line (yellow), is moved further towards the 

 violet end, and is at about 530 /Z/u- instead of at about 580 /^t/u,. The 

 luminosity curve falls slowly towards the violet end, and sharply 

 towards the red ; the red end is shortened. It may be stated at once 

 that all types of colour blind give almost identical achromatic scotopic 

 curves^. Hence we can consider here results obtained both by normal 

 observers and by colour-blind observers like Nagel* (a deuteranope). 

 Very accurate observations were made by Schaternikoff^ in v. Kries's 

 laboratory. Fig. 15 shows the similarity between Nagel's (deuter- 

 anopic) and Schaternikolf's (normal) curves. 



Fig. 16 shows the sunlight, Nernst light, and gas light curves^. 



The summits of the gas light curve (537"2/x/x) and the sunlight 

 curve (529'3;u./x) differ slightly, and light from cloudless sky gives 

 a rather higher value in the green-blue and blue than direct sunlight. 

 An Auer lamp, which is rich in green rays, will give a slightly different 

 curve from a carbon filament electric lamp, which is rich in red rays. 

 The curve will depend upon the energy distribution in the given 

 spectrum, and of course the diffraction or interference spectrum curve 



1 Siizungsher. d. Wiener AkacL, math.-naturw. Kl. xcviii. 70, 1889. 



2 Phil. Trans. Roy. Soc. Lond. clxxxiii. A, 531, 1892. 



^ Raehlmann, Zlsch. f. Augcnlilk. ii. 31.5, 403, 1899, for a possible exception ; also 

 Tschermak, Ergeh. d. Physiol, i. 2, 703, 747, 1902. 



* V. Kries and Nagel, Ztsch.f. Psychol, u. Physiol, d. tiinnesorg. xii. i, 1896. 

 5 Ibid. XXIX. 2fi5, 1902. 



* Trendelenburg, Ztsch.f. Psychol, u. Physiol, d. Sinnesorg. xxxvn. 1, 1904 



