OPTICAL PROJECTION 



may be a convenience ; but few would question that for mere 

 effectiveness, in such an experiment as this, a larger ball of 

 ivory, clearly seen in its rebound from a metal plate, is to be 

 preferred. 



Take again the propagation of a wave in an elastic medium, 

 as illustrated by the well-known apparatus shown in fig. 104. 

 Using small glass balls, with wires for the suspending frame, 

 this is very easily brought within the field of the lantern, the 

 apparatus not exceeding, say, 4 inches in total width. That 



may be a great con- 

 venience and advan- 

 tage, in storage or 

 portability. But if 

 not, then it becomes 

 a question whether 

 it is worth while 



to project apparatus 

 FIG. 104 



made on such a 



miniature scale ; or whether larger apparatus, or a row of 

 billiard balls in a trough, shall be employed in the usual 

 manner, in view of the auditory. 



Ail questions of adapting physical apparatus for projection, 

 should be considered from this practical point of view. 



109. Shadow Projections. The screen method of demon- 

 stration is not, however, limited to small apparatus ; a great 

 deal constructed on quite a large scale can be projected on the 

 screen by a method I first found described by Professor 

 Dolbear * in connection with the heliostat, and which I have 

 found, with a very little modification, to be quite efficient with 

 the lantern. Professor Dolbear condenses the solar beam by 

 a lens of short focus, and in the rays diverging from this 

 focus as from a point, the apparatus is placed, casting a very 

 sharp shadow upon the screen. To show the immense range 

 of this method, let us take the very simple apparatus illu*- 



1 The Art of Projecting. Boston: Lee & Sheppard. 



