248 



OPTICAL PROJECTION 



depend upon motions, and especially rhythmical vibrations, 

 they are for the most part capable of being thus demonstrated. 

 In some cases projection is upon the whole less effective 

 than the direct observation of large apparatus, but may 

 enable the phenomena to be shown with smaller and less 

 expensive apparatus : in others there is perhaps little choice : 

 in others projection is distinctly superior, or may be the only 

 method practically available. 



134. Wave Motion. The best method of showing the 

 nature of the motion of the particles of air in a sound-wave, 



is to project it by 

 what is usually 

 called a Crova's 

 disc, constructed 

 as follows. A cir- 

 cular transparent 

 plate 13 to 18 

 inches diameter is 

 so mounted on an 

 axle that it can 

 be rotated with 

 one of its hori- 

 zontal radii across 

 the condenser- 

 field, and the 

 radial portion 

 there focussed as 



a slide. The plate is blackened, and in the centre is struck a 

 circle 6 to 8 mm. diameter, which is divided into twelve divi- 

 sions, as fig. 181. Taking a scale divided into, say, eighths of 

 an inch (it may range from one-twelfth to one-eighth accord- 

 ing to size), take a radius of from 2 to 3 inches, and strike a 

 circle through the varnish, from one of the divisions on fig. 131, 

 cutting a pretty bold transparent line (too fine a scratch will 

 not be bold enough upon the screen). Extend the compasses 



FIG. 130. Orovs's disc 



