1 104 VISION, 



deduction from the theory. A second feature is the absence of the 

 photochromatic interval for coloured light of low intensity. According 

 to some observers, the interval is only diminished, and v. Kries supposes 

 it possible that the colour of a light may not be perceived at very feeble 

 luminosities when it acts on the chromatic apparatus proper, although 

 the interval in this case is very much less than that which exists where 

 visual purple is present. A third feature in accordance with the 

 theory, is the absence of Purkinje's phenomenon. This rests not only 

 on the observations of v. Kries and other advocates of the theory, but 

 also on the independent observation of He ring, that the phenomenon 

 may be produced by going from the centre to a more peripheral part of 

 the visual field. According to Tschermak, both Purkinje's phenomenon 

 and the deviations of matches occur in central vision, but to a less 

 marked degree than elsewhere. 1 Nagel and Stark (colour-blind) have 

 found that the deviations from Newton's law do not occur for the 

 fovea, and they have been able to measure the area of the visual 

 field within which colour equations hold good at all intensities, and 

 find that it corresponds closely with the measurements of the rod-free 

 area made by Koster. The existence of an absolute scotoma in total 

 colour-blindness has already been mentioned. Lastly, according to v. 

 Kries, the recurrent image cannot be seen in an area of the visual field 

 corresponding to the fovea. 



This theory of the function of visual purple has been criticised by 

 Hering and Hess, 2 who have objected to the theory mainly in regard to 

 its proposed explanation of total colour-blindness. In two cases of total 

 colour-blindness examined by themselves, and in one examined by 

 Pfliiger, no central scotoma was found. In view of the considerable 

 individual differences which exist in the distribution of rods and cones 

 within the macular area, of the difficulty of detecting a small central 

 scotoma, and of the possibility that in such abnormal individuals the 

 absent or defective cones may be replaced by rods, the failure to detect 

 a central scotoma in some cases does not tell seriously against the 

 theory. Hering and Hess also found in one case of total colour-blind- 

 ness that, when dark-adapted, the sensitiveness increased from the 

 centre to the periphery of the fovea, as in the normal eye. This fact is, 

 however, not inconsistent with the theory. Hering and Hess also 

 found that one of their cases, a good observer, saw a recurrent image. 

 Uhthoff 3 has since given a full description of a case in which there was 

 a definite central scotoma, and other features which were all in harmony 

 with the hypothesis that vision depended on visual purple. 



Some of the peculiarities have been referred to the influence of the 

 pigment of the macula, which probably extends throughout the fovea. 

 Its absorptive power might explain the diminished sensitiveness to light, 

 though, according to Breuer, the diminution is too great to be accounted 

 for in this way. 



The accumulation of evidence in favour of this theory of the function of 

 visual purple is further supplemented by the comparative evidence which first 

 led Schultze in this direction. Krause 4 has found that Schultze was wrong in 

 supposing that rods were exclusively present in nocturnal animals, cones being 



1 Arch.f. d. ges. PhysioL, Bonn, 1898, Bd. Ixx. S. 297. 



2 Ibid., 1898, Bd. Ixxi. S. 105. 



3 Ztschr. f. Psychol. u. Physiol. d. Sinnesorg., Hamburg n. Leipzig, 1899, Bd. xx. S. 326. 



4 Internat. Monatschr.f. Anal. u. PhysioL, Leipzig, 1886-1895. 



