156.]
§21. On the Intensity of the Light Sensation
187
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