SCOTOPIA OR TWILIGHT VISION 



59 



than the red without bringing about any change in intensity. Even 

 momentary adaptation on lowering the illumination elicits the Pur- 

 kinje phenomenon. These experiments cannot be regarded as con- 

 clusive, partly on account of the different amounts of white light 

 transmitted by the coloured glasses, but undoubtedly adaptation plays 

 the preponderant role^. 



As might be expected from our previous experiences of luminosity 

 phenomena the scotopic relative increase in brightness is associated 

 with a diminution in saturation. Those red rays which have little 

 (620 — 670jUjLt) or no (beyond QTO/jl/jl) scotopic stimulus value, show no 

 change. 



The eft'ect of Purkinje's phenomenon is seen graphically in Fig. 18, 

 where the abscissae represent intensities of red lights (670 /x^) and 

 the ordinates those of blue light (450 fifx) ; points on the curve represent 

 equivalent luminosities^. The deviation from a straight line at low 

 intensities shows the relative increase in brightness of the blue light. 



10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 



Fig. 18. Curve of equivalent luminosities of red (670 /xfj.) and blue (450 fxfx) lights at difiFerent 

 intensities (slit-widths). (Konig.) 



Although different colours alter their appearance on diminution of 

 their intensity it might be thought that accurate photopic matches 

 would remain valid for the scotopic eye. This, however, is not the case. 

 Albert^ noticed that spectral yellow when matched with a suitable 

 mixture of red and green ceased to match on diminishing the intensity, 

 the mixture becoming brighter and less saturated than the homogeneous 



^ Cf. Feilchenfeld, Ztsch. f. Sinnesphysiol. XLiv. 51, 1909. 

 ^ Konig and Brodhun, in Konig, p. 144. 

 ^ Wiedemann's Ann. xvi. 129, 1882. 



