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
§21. On the Intensity of the Light Sensation 
1. Bright areas appear magnified. The dimensions of narrow 
apertures and slits illuminated by light from behind are never es¬ 
timated correctly. They invariably look wider than they really are, 
even with the most perfect accommodation. Similarly, too, the fixed 
stars seem to be small bright surfaces, even when we look at them 
through a concave glass in order to be able to accommodate exactly. 
In a grating of fine dark bars with intervals exactly as wide as the bars 
themselves (ordinary wire grating for interference experiments), 
held in front of a bright background, the intervals appear to be wider 
than the bars. When, in addition, the accommodation is not perfect, 
the phenomena are much more striking and are visible even with 
larger objects. Fig. 32 shows a black square on a white background 
alongside of a white square on a black background. With good 
illumination and insufficient accommodation, the white square appears 
larger, although they are both equal. 
Fig. 32. Fig. 33. 
2. Adjacent bright areas tend to flow over into each other. A fine 
wire held between the eye and the sun or a bright flame disappears, 
because the two bright surfaces adjacent to it in the field of view 
encroach on it from both sides and fuse together. With patterns 
composed of white and black squares like a chess-board, as shown in 
Fig. 33, the white squares fuse by irradiation at adjacent corners and 
separate the black ones. Plateau used squares of this sort also to 
measure the spread of irradiation. The white fields were cut out of a 
dark screen and illuminated from behind. One of the two black squares 
could be shifted horizontally by a screw, and was adjusted so that the 
two middle vertical lines appeared to the spectator to coincide in one 
line. For measurements at longer distances the black fields were 
made of little boards, but for shorter distances they were made of 
fit tie steel plates. The error made in adjusting the square was called 
the spread of the irradiation. 
3. Straight lines become broken. If the edge of a ruler is held be¬ 
tween the eye and a bright flame or the sun, it seems to have a break 
in it where the bright object protrudes above it, as represented in


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