SOUXD 253 



the height of the optic axis, and the rays from the condensers 

 slightly converged so as to be condensed into the space that 

 will be occupied by the transverse vibrations of the bead, 

 which must never leave the illuminated area, but otherwise 

 the more light is condensed upon it the better ; an area 2 

 inches diameter is about the thing. The beam may be 

 managed in several ways. The lantern may be deflected 

 parallel with the screen, or a little more, so that the rays fall 

 nearly at right angles upon the rod, which must always be 

 pointed to the screen ; or the light may be thrown back 

 nearly straight towards the end of the rod by the plane 

 mirror ; the object focussed upon the screen with the loose 

 lens being the luminous spot reflected by the spherical bead, 

 this will be equally bright, and the apparent motion the same, 

 either way. The surplus light should, however, be stopped by 

 a black screen if convenient. The rod may now be bowed 

 near its base with a violin bow well resined, or struck at 

 various points with a wooden stick ; and by either method all 

 possible varieties of beautiful harmonic scrolls will appear 

 rippling over the screen in lines of light. 



138. Tuning-Forks. The vibration of a common c 256 fork 

 is easily projected in shadow upon the screen. Or it can be 

 impressively shown by focussing in the field of the lantern a 

 small glass filled with water to the brim. On striking the 

 fork, and touching the surface of the water with both vibrating 

 prongs, water will be splashed off in both directions, pro- 

 ducing much more effect on the screen than would be 

 supposed. 



Employing sympathetic vibrations excited by another fork 

 in unison, the vibration of a fork can be projected by the 

 simple arrangement shown in fig. 133, consisting of a rect- 

 angular wooden framework on which is reversed (i.e. the fork 



through the bead, should be heated in a Bunsen burner, and bent at right 

 angles, and the bead fastened on this portion with the glue. This applies to 

 all the rods subsequently mentioned. 



