322 



OPTICAL PROJECTION 



the grating. In the top and bottom edges of the slide are 

 cut longitudinal grooves, B B, the whole length ; and in these 

 grooves slides a panoramic strip of glass about 18 inches long 

 which is also black-varnished, and has the form of the wave 

 traced boldly through it. On drawing this sliding-piece 

 along, the wave will appear in motion ; and if one or two of 

 the perpendicular scratches be distinguished by extra width, 

 or a wash over it of transparent colour, it can be pointed out 

 upon the screen that every single dot only moves up and 

 down, and that the wave-motion consists in the similar 



motions of successive dots 

 being a little later in time. 



I constructed another 

 single -wave slide in another 

 manner, bending a piece of 

 wire round a glass tube into 

 a helix, about |-inch dia- 

 meter and f -inch pitch. Re- 

 moving the core-tube, the 

 straight ends of the wire were 

 then brought in to coincide 

 with the axis of the helix, 

 and the whole mounted in 

 a wooden frame so as to be 



revolved by a small winch-handle, in an aperture just a little 

 larger than allowed the whole helix to be seen in profile. 

 This being placed in the stage and projected on the screen, 

 on turning the winch an apparently plane wave in "black 

 appears in motion, which may be continued ad libitum. By 

 affixing two or three bits of very fine wire as spurs here and 

 there, or a morsel or two of wax, the purely transverse motion 

 of any given point may be shown in this slide also ; and it 

 may further be used to illustrate the propagation of a circular 

 wave. 



J84. Interference of Waves. The slide partly shown in 



FIG. 176. Wave Slide 



