SCOTOPIA OR TWILIGHT VISION 57 



the size of a postcard, according to their apparent brightness. Red 

 and green will occupy the dark and light ends of the series and orange 

 will match a dark blue. Similar experiments can be made to give 

 quantitative estimates with the colour top. 



The explanation of these facts is found in the scotopic luminosity 

 curves. In the bright prismatic spectrum of gas light the red (670 /x/x) 

 appears about ten times as bright as the blue (480 ixfju). On the other 

 hand the achromatic scotopic value of the red is less than one-sixteenth 

 that of the blue. Hence with failing light the brightness of different 

 coloured objects alters, the colours towards the red end of the spectrum 

 becoming relatively darker, those towards the violet end brighter, so 

 that finally the reds appear almost black and the blues bright. This 

 fact was first investigated by Purkinje (1825) and is known as Purkinje's 

 phenomenon. Hering^ drew attention to the fact that the brightness 

 of the blues increases much more rapidly than that of the reds diminishes. 

 This depends upon the condition of dark adaptation of the retina. 

 K5nig'2 {v. p. 45) paid more attention to variations in the intensity of 

 the light. The dift'erences of these two authors on the subject of 

 Purkinje's phenomenon depend upon the fact that two factors have to 

 be considered — dark adaptation and stimulus-intensity {vide infra). 



It is to be noted that in every-day experience we are not dealing with 

 complete dark adaptation. The scotopic luminosity curves given above 

 apply only to very thorough dark adaptation. In such experiments 

 as Konig's (Fig. 10) the adaptation was certainly changing, though 

 there must have been a fair degree of dark adaptation, and the main 

 alteration in the form of the curves is rightly attributed to intensity 

 of stimulus. It will be seen from these and other such curves that the 

 shift in sensation intensity with diminished stimulus intensity is gradual. 

 I wish to avoid as far as possible introducing theoretical considerations 

 into this part of my book, but it will simplify comprehension of the 

 subject if it is pointed out at once that the easiest explanation of the 

 gradual shift of the curves is the simultaneous action of two processes, 

 one especially related to vision with relatively high stimulus values and 

 light adaptation, the other to vision with low stimulus values and dark 

 adaptation. Upon this explanation scotopic vision is a relative con- 

 dition, its nature depending upon low^ stimulus values and a com- 

 paratively high degree of dark adaptation. Whether the processes are 

 subserved by independent physiological mechanisms is a question to be 



^ Arch.f. d. ges. Physiol, i.x. 51G, 1895. 2 Konig, p. 114. 



