AMUSING SCIENCE.

Otago Witness, Issue 2046, 11 May 1893, Page 38

AMUSING SCIENCE.

Persistence of Luminous Impressions on the

A great number of optical illusions are due to the persistence of luminous impressions on the retina. We all know the phenomenon which takes place when a stick, one end of which is on fire, is made to revolve rapidly in a darkened room. The impression is that a luminous circle is revolving. A number of these interesting experiments may be made by tops specially designed for the purpose. A galvanised iron wire, bent to the outline of some common object— a drinking glass for example— is affixed to the axis of the top, and as the top revolves rapidly the outline of wire leaves the impression of a perfect glass. With the help of an empty nutshell and the handle of a pen, a little mill with a vertical axis may be constructed, by means of which all the illusions of the luminous top may be produced. Out the nutshell in two, and cut a groove at the top and bottom of each half. Carefully round the middle of the pen handle— which is to form the axis— to fill these grooves. The rounded part must be slightly narrower than the top and bottom projections. Square off the projecting top end, and affix to it, some distance downabout midway between the extremity and the top of the nut— a large disc of pasteboard. Roll a string round the centre of the axis and lead it through a hole bored in the side of the shell. The half-shells should now be glued firmly together. By holding the nutshell in one hand and pulling the string a rapid rotatory movement is imparted to the axis. Hollow out the top of the pen handle with a hot hon wire, and in the hollow thus made the end of the wire which has been bent to the desired shape may be affixed. The outline of a drinking glass, a lamp glass, or many other shapes may be used, and the wires may be painted a colour most suitable.