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 



