178
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