268 OPTICAL PROJECTION 



all within a few inches compass, upon one base, at a very 

 small expense. 



146. Vibrations in Air, Membranes, and Plates. Any 

 membrane which is pretty transparent, vibrated in any 

 manner so as to show the nodal lines by powder strewn on 

 its surface, can be projected in the vertical attachment or 

 lantern. Goldbeaters' skin is clear, but the wrinkles in it 

 project as lines, and on the whole, varnished vegetable parch- 

 ment answers best ; unless it be a very thin film of mica, 

 which is flat, thin, quite transparent, and best of all. 



The vibration of a membrane covering a capsule into 

 which sound is conveyed by a tube, may be shown in several 

 ways. The simplest is to adjust the capsule so that the 

 membrane stands in a vertical plane, and to hang against it 

 a light pith ball. The ball will be thrown away when the 

 membrane vibrates, and this can easily be projected. 



A more elegant method is what Professor Dolbear has 

 called the opeidoscope. The membrane may be stretched 

 over one end of a mere pasteboard tube, into the other end of 

 which notes are spoken or sung. To its centre is cemented a 

 small mirror of silvered micro-glass. This receives a pencil 

 of rays from the lantern, which after reflection is focussed as 

 a spot upon the screen as usual. Upon singing into the tube 

 the spot will describe a figure, which may possibly take any of 

 the Lissajous forms, though this is rare except as to the circle 

 or oval. Sometimes the mirror will not happen to be placed 

 ' happily ' on the membrane to show figures ; in that case, 

 pressure near the edge of the membrane with the end of a 

 knife, will probably so shift the nodes and vibrating portions 

 as to remedy this. For my own use, I have devised an 

 instrument which is very handy in such experiments. A 

 brass tube about 4 inches long and 2 inches diameter is 

 tapered to an inch at one end to receive strained over it a 

 rubber speaking-tube, at the other end of which is a mouth- 

 piece ; and to its centre is attached a stud, by which it is fixed 



