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.
Ice machines are constructions designed to employ the heat generated from coal in extracting the heat stored up in water at the ordinary temperature. One ton of coal will make 15 tons of ice, and yet only about 1 per cent. of the power used is utilized, these machines being especially wasteful of heat. The work is done through the medium of some volatile fluid, like ether or ammonia, or by the use of previously cooled air. Raoul Pictet, who advocates the employment of another fluid--sulphurous acid solution--says that every machine must comply with five conditions: 1. Too great pressure must not occur in any part of the apparatus. 2. The volatile liquid employed ought to be so volatile that there will be no danger of air entering. 3. It is necessary to have a system of compression which does not require the constant introduction of grease or of foreign materials into the machine. 4. The liquid must be stable, it must not decompose by the frequent changes of condition, and it must not exert chemical action on the metals of which the apparatus is constructed. 5. Lastly, it is necessary, as far as possible, to remove all danger of explosion and of fire, and for this reason the liquid must not be combustible. The only substance, in his opinion, that answers these requirements is sulphurous acid. This subject is a very important one. If the utilization of heat could be carried to 3 per cent., as in most machines, it might be possible to make ice cheaper in New York than to gather, store, and transport it.