Volume 23 — CURIOUS OPTICAL EXPERIMENTS.

Prof. Nipher of the Washington university at St. Louis describes some optical illusions, easily tried and apparently very singular, as follows: 1. Fold a sheet of writing paper into a tube whose diameter is about three cm. Keeping both eyes open, look through the tube with one eye, and look at the hand with the other, the hand being placed close by the tube. An extraordinary phenomenon will be observed. A hole the size of the tube will appear cut through the hand, through which objects are distinctly visible. That part of the tube between the eye and the hand will appear transparent, as though the hand was seen through it. This experiment is not new, but I have never seen it described. The explanation of it is quite evident.

2. Drop a blot of ink upon the palm of the hand, at the point where the hole appears to be, and again observe as before. Unless the attention be strongly concentrated upon objects seen through the tubes the ink-spot will be visible within the tube (apparently), but that part of the hand upon which it rests will be invisible, unless special attention be directed to the hand. Ordinarily the spot will appear opaque. By directing the tube upon brilliantly illuminated objects, it will, however, appear transparent, and may be made to disappear by proper effort. By concentrating the attention upon the hand, it may also be seen within the tube (especially if strongly illuminated), that part immediately surrounding the ink spot appearing first.

3. Substitute for the hand a sheet of unruled paper, and for the ink spot a small hole cut through the paper. The small hole will appear within the tube, distinguishing itself by its higher illumination, the paper immediately surrounding it being invisible. Many other curious experiments will suggest themselves. For example: if an ink spot somewhat larger than the tube be observed, the lower end of the tube will appear to be blackened on the inside.