Scientific amusements . at figure are parallel witheach other, but they appear convergent and divergent,and seem constantly turned out of a vertical position intoa direction inverse to that of the oblique lines whichdivide them. The separate halves of the oblique linesare displaced respectively. If the figure is turned so thatthe broad vertical lines present an inclination of 45° tothe horizon, the convergence appears even more remark-able, whilst we notice less the apparent deviation of thehalves of the small lines, which are then horizontal and 74 OPTICAL ILLUSIONS. vertical. The direction o
Scientific amusements . at figure are parallel witheach other, but they appear convergent and divergent,and seem constantly turned out of a vertical position intoa direction inverse to that of the oblique lines whichdivide them. The separate halves of the oblique linesare displaced respectively. If the figure is turned so thatthe broad vertical lines present an inclination of 45° tothe horizon, the convergence appears even more remark-able, whilst we notice less the apparent deviation of thehalves of the small lines, which are then horizontal and 74 OPTICAL ILLUSIONS. vertical. The direction of the vertical and horizontallines is less modified than that of the oblique lines. Wemay look upon these latter illusions as fresh examples ofthe aforesaid rule, according to which acute angles clearlydefined, but of small size, appear, as a rule, relativelylarger when we compare with obtuse or right angleswhich are undivided ; but if the apparent enlargement ofan acute angle shows itself in such a manner that the two. The vertical strips are parallel ; they appear convergent or divergentunder the influence of the oblique rays. sides appear to diverge, the experiments will result inillusions given. On page 73 the two halves of each of the two straightlines .seem to deviate through the entire length in such amanner that the acute angles which they form with theoblique lines appear enlarged. The same effect is shownby the vertical lines of above illustration. M. Helmholtz is of opinion that the law of contrast jsinsufficient to entirely explain the phenomena, and believes Experiments. 7e that the effect is also caused by the movements of theeye. In fact, the illusions almost entirely disappear, ifwe fix on a point of the object in order to develop anaccidental image, and when we have obtained one verydistinctly, which is quite possible with Zollners design,this image will present not the slightest trace of displacement of the gaze will exercise no verydecided influ
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