266 OPTICAL PROJECTION 



143. Kaleidophonic Figures. The usual Lissajous' fig- 

 ares, but not beats, can be projected by Wheatstone'a 

 kaleidophonic method. There are three ways of preparing 

 rods in order to produce them. The most certain, and the 

 best where frequent demonstrations are in view, is to prepare 

 rectangular steel rods, which vibrate in the requisite propor- 

 tions. Square steel about 2mm. square is always procurable, 

 as also is rectangular steel about T V x J inch in section. 

 By filing down these and mounting each with a bead, all the 

 intervals can readily be obtained. Secondly, a piece of thin 

 round rod may have several inches at the top end bent 

 back into a loop, like a lady's hair-pin. Cementing a bead 

 on to the bend at the tip, by screwing in the vice so that 

 variable lengths of the longer stem project, various figures 

 will be obtained ; but they are not so true as by the previous 

 and following method. Thirdly, a straight piece of rathe: 

 thick clock-spring or similar steel is prepared, so that about 

 six inches at the tip may have its section at right angles 

 to the remainder, which should be some sixteen inches long. 

 This may be done by cutting the steel in two, and making a 

 saw-cut half an inch down the middle of the long piece, into 

 which notch the other piece fits and is brazed ; or the steel 

 may be heated red-hot in a Bunsen burner just at that point, 

 and twisted sharply round at right angles with a pair of 

 pliers. The top end should be somewhat tapered, and tipped 

 with either a bead, or a segment of a hollow sphere of bur- 

 nished aluminium, which will give bolder projections. By 

 nipping the longer stem in the vice at various points, which 

 can be marked, all the intervals can be produced ; either with 

 a bow, or by striking the rod, they may be shown either with 

 or without ha,rmonic vibrations superposed. 



144. Vibrations of Strings. For showing the various 

 loops described by a string at different points, there is no 

 method to equal Tyndall's, of employing a thin ribbon of 

 planished silver or other metal aluminium would now be the 



