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 



