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272 OPTICAL PROJECTION is a great advantage. Sometimes a black card or other screen may need to be adjusted at the side next the screen, to prevent any but the reflected rays which form the image from passing the apparatus; this will suggest itself. The mouth-piece I have found it advisable to construct as in the section (fig. 143), with a membrane of thin india-rubber across it. TKis does not interfere with the true sound vibrations, while it prevents the film from being prematurely rup- tured by any actual blast of air which might result from unskilful management of the voice. Recipes for the solution will be found in Chapter XX., and a good film will often last a quarter of an hour. Besides the usual apertures, a hexagonal one should be provided; for these about two inches in diameter is a good size. The beautiful figures, and the gradual change in colour, make this one of the most fascinating of all lantern experiments. Withdrawing the funnel and laying larger plates with apertures and films upon the box itself, beautiful figures may also be obtained by exciting a tuning-fork, and simply holding the open end of the resonance-box towards the open side of the box under the film. These figures are very powerful in their vortex motions, and perfectly stable. A cornet blown towards the box under such a film will also excite powerful vibrations. A thin film of mica stretched on a frame and dusted with lycopodium, and placed instead of the soap-film, will also show very strong nodal lines. As already remarked, the same apparatus is very handy for exhibiting surface ripples, or in fact any bright surface projection by reflected light. 149. Chladni's Figures.— These may be projected in several ways by the aid of the vertical attachment, using of course plates of glass, with ground and polished edges, (a) A Pia. 143