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Page:Popular Science Monthly Volume 70.djvu/472

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the first, probably not far in front of the retina. On this last class, some beautiful halo phenomena were observed.

When one of these spots was outside a region identified as approximately the yellow spot, it appeared as a circular dark area of some 30′ diameter as shown in Fig. 3. When it came within the yellow spot, it became lighter, and was surrounded by the halo, with its intensified zone and secondary image well defined as in Fig. 4. When, however, it came within the region of most distinct vision, which was very rare, it gave the most beautiful halo effect I have yet seen. It had a dense, black spot in its very center, usually well rayed; then, a light zone limited by an intense black ring, which in turn produced its own complete halo. This form is shown in Fig. 5.

PSM V70 D472 Illusion of the position of a mote in the eye.png
Fig. 5. 'Dot' Mote in Fovea. Fig. 6. Same as Figures 5, viewed at Close Range. Notice different length of rays compared to diameter of ring.

This mote observation is by no means easy. I have often waited fifteen minutes for a mote of this type to appear, and only once have I kept one in sight for any length of time. It then remained in the center of vision for at least twenty minutes. Usually, they float past the center of the vision and give one only a brief view. The size of pin hole used is 1/50 inch. With a much larger hole, say 1/20 inch, they become blurred. By getting near a large lamp shade so that a wide angle of light is viewed, they are best discovered. Then one may retreat from the light and view them as illustrated in Figs. 3, 4 and 5.

The rays observed in the central spot are very interesting. Their length offers a means of measuring the height of the spot above the retina. A short calculation upon approximate data results in 0.002 inch as the distance of the spot from the retina.

It is true that these mote observations require great patience, but