Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
The Sensations of Vision 
[148, 149. 
Ho of the intrinsic light would be measured in terms of an objective 
light. Volkmann made some experiments and found that the in¬ 
tensity of the intrinsic light H0 was equal to the luminosity of a black 
velvet surface illuminated by a tallow candle at a distance of 9 feet. 
The discrepancy between the law and the facts for the upper 
limits of luminosity may be attributed to the fact that the visual organ 
begins to be impaired, as Fechner supposes. The internal changes 
in the nerves that have to communicate the impression of the stimulus 
to the brain cannot exceed a certain definite limit without destroying 
the organ; and hence every action of the stimulus has an upper limit 
set for it, to which must necessarily correspond also a maximum 
intensity of sensation. 
But, moreover, whatever may be the circumstances that tend to 
upset the validity of Fechner’s law at the upper and lower limits of 
luminosity, the same conditions make their influence felt in accurate 
measurements even with medium degrees of luminosity; although, of 
course, that is no reason why the law should not still be regarded 
as being a first approximation to the truth. Unquestionably, most 
paintings, drawings and photographs of ordinary objects can be seen 
equally well under very different degrees of illumination. And yet in 
some photographs the writer has discovered gradations of shade which 
do not come out perfectly clear except for a very definite intensity of 
light. This is particularly noticeable in pictures of landscapes in which 
far distant chains of mountains are represented as half floating in cloud. 
But, so far as the writer is concerned, the most striking instance of this 
peculiarity was in the case of some stereoscopic views of Alpine scenery 
photographed on glass, which showed parts of glaciers or peaks 
covered with snow. By lamplight or moderately bright daylight such 
surfaces of snow look like uniformly 
white areas; but when they are turned 
towards the bright sky delicate shades 
appear, indicating a sort of moulding of 
the white fields of snow; and then they 
disappear again with still brighter fight. 
Of course, delicate shades of this kind 
are found in photographs simply by acci¬ 
dent; and in paintings and drawings 
they are unexpected. But the rotating 
disc affords an easy way of producing 
very delicate shades, of any desired 
luminosity as compared with the white 
background. Masson also has already made use of them for photo- 
metrical experiments. These shades are easy to get by making a


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