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132 SCIOPTICON MANUAL. good window glass, of the size used for magic lantern slides (I prefer 3x4), some tin-foil, such as paper-hangers paste on damp walls before papering, and some paste made of gum tragacanth; with a sharp knife, laying the foil on a plate of glass, the arrow-shaped opening can be* readily cut, and its edges will be as smooth as the most skilful mechanic can make a brass plate. This foil, so prepared, should be mounted between two slips of glass, and the edges bound with paper. Gum traga- canth will cause paper to adhere to glass very firmly and is a nice, clean paste to use. The slide thus prepared will be found to be quite as good as the most costly one procurable in the stores. In my own experiments, when I require slits or openings of any required shape, in opaque plates, I have invariably made them in this manner, with a feeling of satisfaction at their cheapness. A very convenient device to show wave motion can be made with this tin foil. One slide is made with plates of glass, 3x4 inches, having tin foil inclosed, in which slits are cut crossways, say ^ 5 inch wide, 2 inches long, and the slits placed i of an inch apart. I have sometimes pasted slips of tin foil i of an inch across the plate, at equal distances, say T ^- of an inch, in preference to cutting them in a solid piece of foil. This slide will show ver- tical bars of light on the screen. If now another slide bo made of two glasses, 3x6 inches, with foil between them, in which foil a wave-like opening be cut, say i of an inch wide, this slide of itself would show in the lantern a wave line of white on a dark ground on the screen. The two slides put together in the lantern will show a wave line of dots, and if the wave-line slide, which is twice as long as the one with bars, be moved back and forth in front of the bars, the dots will seem to rise and fall in wave motions, and the fact will be demonstrated, that in wave