io8 4 VISION. 



sensibility is the same in the central (except fovea) and peripheral retina, 

 and also in different parts of the latter. 



Some observers have employed the threshold as the test, and others 

 the just observable difference, while Guillery has used the extent of the 

 retinal image which enables discrimination of difference. No observer 

 has compared the threshold and difference methods, both with and with- 

 out dark-adaptation. 



According to Schon, 1 there is a marked difference between the light 

 sensibility of the nasal and temporal halves of the retina. He found 

 that a point 45° from the fovea on the temporal side required, for the 

 production of a sensation of a certain degree of brightness, three times 

 as much light as the corresponding point on the nasal side. Guillery 

 has also found the temporal half less sensitive, but the difference he 

 describes is very much less than that of Schon. Neither observer 

 mentions the conditions of adaptation. 



Exner and Bellarminow have found that nicker is more marked 

 in the periphery than in the centre of the visual field, and have in- 

 ferred from this that the sensation produced by a definite stimulus is 

 there more intense. Bellarminow, further, has found that flicker occurs 

 more readily with stimulation of the nasal than of the temporal half, 

 thus supporting the view of Schon. Schon has advanced a theory of 

 binocular vision founded on this difference between the temporal and 

 nasal halves of the retina (p. 1138). 



Colour-sense of periphery.— The colour vision of the peripheral 

 retina has been investigated by many observers with very divergent 

 results. The divergency depends to a very great extent on neglect of 

 such factors as adaptation, the brightness and degree of saturation of the 

 coloured light employed, the size of the area stimulated, and the nature 

 of the background. The sensibility is generally tested by passing over 

 the retina from periphery to centre, or vice versd, patches of coloured 

 light, and noting, first, the limits where the different colours begin 

 or cease to be perceived ; and, secondly, the changes in colour-tone, 

 saturation, or brightness which the patches appear to undergo. In 

 general, it may be said that the brighter and more saturated the colour, 

 and the larger the area stimulated, the greater is the distance from the 

 centre at which a colour can be recognised. It is generally agreed that 

 at ordinary intensities no colours are perceived at the extreme periphery, 

 all being perceived as some shade of grey, depending on the kind of light 

 and on the background; but, according to many observers, including 

 Landolt 2 and Charpentier, 3 all colours can be recognised as far peripherally 

 as white, if the light is made sufficiently intense. 



According to Hering, the grey with which a given colour is matched at 

 the extreme periphery of the retina is the same as the grey with which it 

 is matched at a low intensity, and he has used this match as one means of 

 determining the white value of a colour, v. Kries 4 has latelv determined 

 the white value of both pigment and spectral lights for the periphery of 

 the retina and for the dark, and found a great difference between the 

 two, the periphery values corresponding fairly well with the normal 

 brightness values by heterochromatic photometry. 



1 Arch./. Ophth., 1876, Bd. xxii. Abtli. 4, S. 49; 1878, Bd. xxiv. Abth. 1, S. 27. 



2 Klin. Monatsbl.f. Augenh., 1873, Bd. xi. S. 376. 



3 "La lumiere et les couleurs," Paris, 1888, p. 183. 



4 Centralbl.f. Physiol., Leipzig u. Wien, 1897, Bd. x. S. 745. 



