Volltext: Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition (2)

368 
The Sensations of Vision 
[310, 311. N. 
process of adaptation that it exceeds that of the daylight mechanism for 
the light used in the test, that the conditions are produced for the appear¬ 
ance of the colourless interval. 
In this connection, it should be recalled that the “grey glow” of 
incandescent bodies cannot be seen unless the eye is dark-adapted. For 
the photopic eye the glow is red to begin with. Anyhow the grey glow 
is never seen in the centre of the retina. 
7. Capacity of the Retina for Space and Time Discriminations, in 
Daylight Vision and Twilight Vision 
In ordinary daylight vision the ability to make space discrimina¬ 
tions, or what is usually meant by the “visual acuity”, is greatest in the 
fovea by far. Even a few degrees to one side of the point of fixation it is 
quite a good deal less (see page 34). The approximate form of a curve 
constructed according to Dor’s measurements1 is shown in Fig. 69, 
where the ordi¬ 
nates indicate the 
visual acuity and 
the abscissae de¬ 
note the distances 
in degrees from 
the point of fixa¬ 
tion designated by 
F. According to 
Burchardt2, the 
visual acuity at 
places from 15' to 
20' away from the 
point of fixation 
was equal to the 
maximum value, but at a distance of 30' it was only 80 or 50 percent, 
of the maximum. 
With the enormous superiority of the fovea in the matter of visual 
acuity, it goes without saying that the visual acuity in pure twilight 
vision, where the fovea is excluded as a result of “physiological 
hemeralopia”, will under all circumstances remain far below the 
maximum acuity of daylight vision. Indeed, strictly speaking, visual 
acuity in bright daylight cannot be compared with that in twilight 
vision, because it is dependent on the luminosity of the test chart; in 
other words, the visual acuity diminishes within certain limits as the 
Fig. 69. 
Local variation of the visual acuity of the retina (according 
to Dor). 
1 Archiv, f. Ophthalm. XIX. 3. S. 321. 1873. 
2 Burchardt, Internationale Probe zur Bestimmung der Sehschärfe. Berlin 1893.
	        
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