ADAPTATION OR TEMPORAL INDUCTION 



51 



the fovea after ten minutes dark adaptation, l|- after twenty minutes, 

 and 1 after an hour. The direction is constant for the same eye and 

 varies with different eyes; it depends upon muscular balance and 

 refraction rather than upon the specific sensibility of the parts of the 

 parafoveal region (Simon). 



Although the fovea is night-blind relatively to the periphery it is 

 capable of a slight degree of dark adaptation 1 , but the small rise in 

 sensitiveness of the fovea 'is only appreciable after previous very strong 

 light adaptation, such as looking at the open sky. 



Breuer and Pertz 2 showed that the peripheral rise in retinal sensi- 

 bility in dark adaptation is rapid from 1 to 4 around the fovea, then 

 slower to a maximum between 10 and 20 beyond which it falls. This 

 is seen graphically in Fig. 13, where the sensibility of the fovea and the 



60- 

 50 

 40 

 30 

 20 

 10 



\ 



\ 



7 



4 



4 3 2 1 



Fig. 13. Sensibility of the fovea and parafoveal region for mixed bluish-white light. 

 Abscissae, to the left of zero degrees to the temporal, to the right, degrees to the nasal 

 side of the fovea (0) ; abscissae, arbitrary scale. (Breuer and Pertz.) 



parafoveal region to bluish-white light are shown. The abscissae to 

 the left represent the temporal side, to the right the nasal. Pertz's 

 experiments show that the scotopic fovea is more sensitive to red light 

 than the periphery, though the difference is slight. Blue light gives a 

 curve resembling that in Fig. 13. Yellow shows 'a slight rise in the 

 paracentral area. 



The alterations in sensibility differ according to the size of the area 



1 Charpentier, Arch. d'Opht. iv. 25)1, 1884 ; xvi. 87, 1890 ; Fick, Arch.f. d. ges. Physiol 

 XLIII. 481, 1888; Treitel, Arch. f. Ophth. xxxv. 1, 50, 1889; v. Kries, Arch.f. Ophth. XLII. 

 '?, !>5, 1896; Tscherniak, Arch. f.d. ges. Physiol. LXX. 320, 1898; Bloom and Garten, 

 ibid. LXXII. 1898. 



2 v. Kries, Ztsch.f. Psychol. u. Physiol. d. Sinnnsorrj. xv. 327, KS'lT. 



42 



