io88 VISION. 



blue field loses greatly in intensity on approaching the fovea, while a similar 

 red field does not. Hering l has found that small fields of red and green, 

 which appear equally bright when fixed centrally, become of different 

 brightness when observed indirectly, and points out that Purkinje's phenomenon 

 may be produced by a change from centre to periphery of the retina. 



The alteration of a colour match, with diminution of the intensity, does 

 not, according to v. Kries, take place in foveal vision (see pp. 1090 and 1104). 

 Hering 2 has 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. Another feature of foveal vision, accord- 

 ing to v. Kries, 3 is the absence of the recurrent image of Purkinie (p 

 1073). 



Wilbrand 4 and Franklin 5 have described in the normal eye the existence 

 of a small blind area in the neighbourhood of the fixation point, its exact 

 position and shape varying in different observers. 



COLOUR-BLINDNESS, ETC. 



Colour-blindness. This is by far the most important abnor- 

 mality of vision, from the theoretical point of view. It is customary 

 to distinguish between congenital and acquired colour-blindness. The 

 former has been more fully studied, and its varieties seem to be better 

 defined, than those due to disease, although for theoretical purposes the 

 latter have advantages that the others lack. Congenital colour-blindness 

 occurs in three chief forms : a rare form, in which all colours are con- 

 fused, usually known as total colour-blindness, or monochromatic vision ; 

 the common form, in which there is confusion between red and green ; 

 and a very rare form, in which yellow is confused with blue. The latter 

 forms are often classed together as examples of dichromatic vision. 



Total colour-blindness. There are now on record a considerable 

 number of cases of this defect, which are undoubtedly congenital. Any 

 colour may be confused with any other, red with green, and yellow 

 with blue. The spectrum is seen as a band of light differing in its 

 different parts in brightness only; and by suitable adjustment of the 

 relative intensities a match may be made between any two parts. The 

 red end is in all, or nearly all, cases considerably shortened, while the 

 violet end has the same limit as to the normal eye. A most striking 

 feature is the nature of the curve of luminosity shown in Fig. 392. The 

 brightest part of the spectrum in nearly all cases 6 is the green about the 

 line E, the actual maximum varying with the source of illumination, while 

 the red end is relatively darker, the violet end relatively brighter than in 

 the normal eye. As Hering 7 was the first to point out, the curve of 

 luminosity corresponds with the curve of luminosity of the normal eye 

 at low intensities and with dark-adaptation. Matches made by Hering 

 with dark-adapted eye were found to be good for his case of total colour- 

 blindness, with the exception of slight differences in the blue and violet, 

 probably referable to slight differences of macular pigmentation. This 

 correspondence has been abundantly confirmed by other observers. 



1 Arch.f. d. ges. Physiol., Bonn, 1895, Bd. Ix. S. 533. 



2 Ibid., 1893, Bd. liv. S. 277. 



3 Ztschr.f. Psychol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 81. 



4 "Die Erholungsausdehnung des Gesichtsfeldes, " 1896, S. 42. 



5 Sitzungsl. d. k. AJcad. d. Wissensch., Berlin, 1894, S. 589. 



6 The most definite exception is a case described by Magnus (Centralbl. f. prakt. 

 Augenh., Leipzig, 1880, Bd. iv. S. 373), in which the maximum brightness was at D. 



7 Arch.f. d. ges. Physiol., Bonn, 1892, Bd. xlix. S. 563. 



