252 OPTICAL PROJECTION 



longitudinal vibrations in a metal rod fixed at its centre, by 

 the repulsion of an ivory ball suspended so as to touch the end, 

 may either be projected entire by the shadow method, or the 

 end of the rod and the ball only may be focussed in the field 

 of the condensers. Another method is to show the action of a 

 glass strip whilst under strain, upon polarised light, as described 

 in 209, page 362. 



The transverse vibrations of rods are easily projected by 

 their shadows alone. For all rod experiments, I have found 

 it most convenient to screw a small parallel vice upon a piece 

 of blackened board, which by a slot and thumb-screw can be 

 adjusted at any height upon another piece of blackened wood 

 furnished with a foot. In such a vice, rods can be fixed at 

 any height, or angle, whether to project their shadows, or 

 beads or mirrors on their ends. A slender knitting-needle 

 projecting 3 to 4 inches will give a very good fan-like shadow 

 when ' sprung ' in the field of the condensers and focussed ; or 

 the shadow method will equally answer for larger rods. 



Nodes and segments are well illustrated by a long piece 

 say two feet of very thin steel, such as clock- spring, slightly 

 loaded at the tip. This being very flexible will show divisions 

 readily when tapped and ' stopped ' at proper intervals ; and 

 the motion being slow, is easily followed on the screen when 

 projected by the shadow method. 



But the superposition of harmonic vibrations upon the 

 fundamental is best projected by Wheatstone's kaleidophonic 

 apparatus. A straight steel wire about 1 mm. or less in 

 diameter, and projecting 8 or 10 inches from the vice, in a 

 nearly horizontal direction, I have found best. To the top 

 is attached a silvered bead about to f-inch diameter, by 

 Prout's elastic glue or otherwise. 1 The bead is adjusted to 



1 The larger the bead, the larger the spot ; but I find this size gives the 

 best results. As I have had some trouble in finding suitable beads, it may bo 

 well to state that they can generally be obtained of theatrical costumiers, such 

 as are found round Covent Garden. Enough of the end of the rod to go 



