84 COLOUR VISION 



than peripheral, but he used too large test objects and thus exceeded 

 the foveal limits (Nagel). More recently a slight increase in the foveal 

 sensibility on dark adaptation has been proved by Wolfflin^ and by 

 Dittler and Koike'^. In the experiments of the latter one eye only was 

 dark-adapted and the luminosities of the binocular double images of 

 the illuminated object were compared. The adaptation at the fovea 

 is so slight that one would not expect to obtain Purkinje's phenomenon 

 there under ordinary circumstances. The demonstration of its absence 

 is beset with difficulties — accurate foveal fixation, comparison of bright- 

 ness of very small areas, etc. — and opinions are therefore divided. 

 Tschermak^, Koster* and Sherman^ are in favour of its presence, but 

 their methods were less satisfactory than those of Konig^, Lummer'^ 

 and Nagel and v. Kries**. v. Kries and Nagel have proved fairly con- 

 clusively that Purkinje's phenomenon is absent over a central field 

 not greater than 2°, i.e., not exceeding the rod-free area. The longer 

 the dark adaptation the better was the delimitation of the area. If a 

 red and a blue spot are fixed with this area, the red being brighter 

 than the blue, then on diminishing the intensity of the light the relative 

 luminosity remains the same. On the slightest deviation of the eyes, 

 however, the blue at once appears the brighter and less saturated. 

 Hering^ found that small fields of red and green, equally bright when 

 fixed centrally, become of different brightness when fixed eccentrically 

 (" Purkinje's phenomenon by change of position in the visual field 

 without change of the intensity of the light "), and he also stated^" 

 that the deviations from Newton's law " become less marked the 

 smaller the area of the retina stimulated, and are absent when the field 

 is made sufficiently small." 



1 Arch.f. Ophth. Lxxvi. 464, 1910. ~ Ztsch. f. Si7inrsphi/sloL XLVI. 166, 1912. 



3 Arch.f. d. ge.s. Physiol, lxx. 297, 1898. * Arch. J. Ophth. xli. 4, 1, 1895. 



» Wundt's Philo.-i. Stud. xin. 1898. « Konig, p. 338. 



^ Verhandl. d. Deutschen. physik. Gcscllschft. vi. 2, 1904. 



^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxili. 167, 1900; v. Kries, loc cit. ix. 81, 

 1896 ; Arch.f. Ophth. xlii. 3, 95, 1896 ; CcntralU.f. Physiol. 1896. 



9 Arch. f. d. ges. Physiol, lx. 533, 1895. " Ibid. Liv. 277, 1893. 



