50 COLOUR VISION 



of the rise of sensibility : santonin has no effect. Mydriatics have an 

 indirect effect ; the first slow rise is prolonged from ten to twenty 

 minutes but is followed by the normal rapid rise to the normal height. 



Very short exposure to bright light, e.g. striking a match, causes 

 a very temporary fall without materially altering the course of the curve. 

 The increase in sensibility after very prolonged dark adaptation is more 

 transient than the increase during the first hour, i.e. it is more quickly 

 and completely abolished by exposure to light. Dark adaptation of 

 one eye has no effect upon the other^. 



Besides this temporal variation in the sensitiveness of the retina 

 there is a well-marked regional variation. In the condition of light 

 adaptation the fovea is the most sensitive part of the retina, though 

 little attention has been paid to the degree of adaptation in the researches 

 published on this subject. (The light sensitiveness of the various parts 

 of the retina must be carefully distinguished from their visual acuity 

 for form.) The regional sensibility for colours of the retina of the light- 

 adapted eye has been worked out by Vaughan and Boltunow^, v. Kries^, 

 and Guillery*. Vaughan and Boltunow found the sensitiveness at 10° 

 from the fovea to be \, at 20° to be y^, and at 35° to be ^f'^^ of that of the 

 fovea itself. In dark adaptation the fovea is the least sensitive part 

 of the retina^. In other words the fovea is a region of physiological 

 night-blindness (v. Kries). 



The relative central scotoma in dark adaptation was long ago recog- 

 nised by astronomers, who noticed that stars of small magnitude were 

 seen better if viewed somewhat eccentrically. " Pour apercevoir un 

 objet tres pen lumineux, il faut ne pas le regarder " (Arago). It is 

 strikingly illustrated in viewing the Pleiades : by direct fixation four 

 or at most five stars are seen ; by indirect fixation a number of weaker 

 stars become visible. Different observers use different parafoveal^ spots 

 for clearest vision in dark adaptation' and the spots vary with the 

 degree of dark adaptation. The nearer the intensity of the stimulus 

 is to the threshold of the dark-adapted fovea the nearer is the spot to the 

 fovea : the feebler the light the more eccentric is fixation. With a 

 given sub-minimal foveal stimulus Simon found that he fixed 2° from 



^ Cliarpentier, La Lumiere et les Couleurs, p. 175, Paris, 1888. 



2 Ztsch. f. Sinnesphysiol. xlii. 1, 1907. 



^ Zl-sch. f. Psychol, ii. Physiol, d. Sinnesorrj. ix. 81, 189fi. 



* Ibid. xii. 261, 189() , xiii. 189, 1897. ^ Donders, Brii. Med. J. 1880. 



* I.e. in the region near the fovea. 



' Christine Ladd-Franklin, in Konig. p. 353 ; Simon, Ztsch. f. Psychol, u. Physiol, d. 

 Sinmsorg. xxxvi. ]8fi, 1904. 



