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Page:Popular Science Monthly Volume 70.djvu/469

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465
ILLUSIONS OF VISION

at the first view, it is a striking phenomenon. I find on the whole that trained eyes are the ones which see it most quickly.

A more beautiful and elegant way of making the experiment is by standing a black-headed hat pin in the middle of a white-walled room, and looking at it against the distant white background. Around the head of the pin will then appear this halo, more beautiful than PSM V70 D469 Vision test dot.pngFig. 1. This Spot should be viewed from a Distance of Six or Eight Feet, with care to avoid fatigue or after-images, in order to see the fine dark halo ring about it at the distance indicated by the smaller dot. before, suspended in mid-air, in the good old-fashioned manner of saintly halos.

The experiment described above gives the 'negative' halo. It will be generally referred to in this article, because it is more easily seen than the 'positive,' The 'positive' form of the halo, however, is most readily seen by a similar method. Let a white-headed pin be substituted for the other, and looked at against a black background. Similarly, a white circle is seen. The difficulties in this case arise from the reflections on the head of the pin and its generally less even illumination.

The effect, however, is the same. Extending all round the head of the pin at a distance of about 7′ of arc (one inch at a distance of 500 inches) is an intensified zone in which the color of the background appears stronger; and outside of that a reduction zone, or ring, or secondary image, in which the intensity of the background is reduced by the addition of some of the color of the spot observed.

In order to find the cause of this halo, many tests were made, of which the first was upon the size of the central spot, It was found that the distance from the edge of the spot to the secondary image is constant; that the width of the secondary image increases to some extent with the size of the spot, and that the intensified area increases its intensification with the size of the spot. If the spot is so small as to be barely visible, the halo may still be seen, but the intensified zone then appears of the same intensity as the background.

If the spot is enlarged sufficiently, both positive and negative halos are seen along its margin, one outside and one inside, so that in a straight line separating light and dark areas, the positive halo may be seen in the dark area, and the negative halo in the light. If two small spots are placed so that their halos intersect, the halo of each