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

N. 311, 312.] 
B. Duplicity Theory and Twilight Vision 
369 
luminosity decreases. And even when the eye is thoroughly dark- 
adapted, the subjective brightness can never be so great as it is in 
daylight vision in bright light, without its rising above the threshold 
of foveal sensitivity and therewith crossing the border between twilight 
vision and daylight vision. However, a comparison may be made be¬ 
tween the visual acuity of a scotopic eye and that of a photopic eye by 
making observations on the two eyes, either when the objective in¬ 
tensity of illumination is the same for both eyes in different states of 
adaptation, or when the subjective brightness is the same. In the 
latter case the objective illumination of the test chart must be much 
smaller for the scotopic than for the photopic eye. Experiments of 
this kind were made by Bloom and Garten.1 When the illumination 
is very low, their results are that the visual acuity of the dark-adapted 
periphery is somewhat higher than that of the photopic eye for the 
same illumination ; but that when the illumination is slightly increased, 
the visual acuity of the photopic eye is the superior of the two. For 
approximately the same subjective brightness, these authors found 
the visual acuity of the scotopic eye was invariably less than that of 
the photopic eye ; but v. Kries2 found it to be the same in the periphery 
for both conditions of adaptation. The essential point is that, whatever 
the differences may be, they are anyhow insignificant as compared with 
the differences between the absolute maxima of visual acuity of the 
photopic eye and the scotopic eye. 
The quantitative connection between visual acuity and the intensity 
of illumination is shown by the measurements made by A. König.3 
The test-objects used in these experiments were Snellen’s hook¬ 
shaped characters, painted black on a white background. The state 
of adaptation always corresponded to the intensity; that is, the more 
complete the dark adaptation was, the weaker was the illumination. 
The results are shown in Table XIII. The figures in the first column 
give the intensity of illumination B, the unit being the intensity of a 
Hefner lamp one metre away. The third column gives the values of 
the visual acuity S expressed in terms of the ordinary Snellen unit. 
It is evident from the table that a curve plotted by taking the intensities 
B as abscissae and visual acuities S as ordinates would not enable us to 
get a very good idea of the connection between these magnitudes, 
1 S. Bloom and S. Garten, Vergleichende Untersuchungen der Sehschärfe des hell- und 
dunkeladaptierten Auges. Pflügers Arch. f. d. ges. Physiol. LXXII. 372, 1898. 
2 J. v. Kries, Über die Abhängigkeit centraler und peripherer Sehschärfe von der 
Lichtstärke. Zentralbl. f. Physiol. VIII. 694,1895. See also : Buttmann, Untersuchungen 
über Sehschärfe. Diss. Freiburg i. B. 1906. 
3 A. König, Die Abhängigkeit der Sehschärfe von der Beleuchtungsintensität. Sits. 
Ber. Akad. Wissensch. Berlin. 13. Mav 1897. S. 559-575; and Gesammelte Abhandlungen 
S. 378.
	        
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