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

N. 312, 314.] 
B. Duplicity Theory and Twilight Vision 
371 
because if the high values of B are to be indicated clearly, the smaller 
values would all be crowded very close together. And so König em¬ 
ployed the convenient method of plotting the logarithms of the in¬ 
tensity as abscissae, which shows the connection between the intensity 
of illumination and the visual acuity in an extremely simple fashion. 
The curve (Fig. 70) is composed of two tolerably straight portions 
which meet each other at an obtuse angle. The break in the curve 
occurs at an intensity of 0.1 metre-candle. Thus, as the luminosity 
increases, visual acuity in- 
12 
U 
10 
0,9 
creases at first in proportion ^ 
to the logarithm of the 
luminosity, and then for a 
short interval the relation 1,3 
is more complicated; but 
soon after the increase be¬ 
comes again proportional to 
the luminosity, but the ratio 
between the two is now 
much larger, as shown by 
the steep portion of the 0,8 
curve. In the equation 
S = alogB the factor a is 
about 10 times as large for 0,6 
the steeper gradient of the 
curve. 
A white surface illu- W 
minated by one-tenth of a 03 
metre-candle is not far from 
n 9 
the threshold of daylight ’* 
vision. Thus the portion of 0,1 
the curve that- lies to the 
left of the abscissa 0.1 corre¬ 
sponds about to the range 
of twilight vision. On the 
other hand, it was stated 
above (page 318) that a white surface illuminated by one metre-candle 
or more was above the threshold of an eye that has been light-adapted 
by bright light. This part of the curve therefore lies definitely within 
the region of daylight vision. As to the intervening segment comprised 
between intensities of one-tenth and one metre-candle, it may be con¬ 
sidered as representing the case of daylight vision that has not been 
dulled by very high intensities of light. Pertz1 found the threshold of 
1 In the article by v. Kries “über die absolute empfmdlichkeit usw.” Zeitschr. f. 
Psychol, u Physiol, der Sinnesorgane XV. 1897. 
0,0 0,00010,001 0,01 0,1 1 10 100 700010000 
Fig. 70. 
Functional connection between visual acuity and 
the logarithm of the intensity of illumination.
	        
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