220 OPTICAL PROJECTION 



of the necessary apparatus, in action, by one or the other of the 

 above methods. It is both impossible and needless to make any 

 pretence of a complete list ; but we will run through a few typi- 

 cal experiments which may serve as specimens or suggestions. 



111. Gravity. Atwood's Machine. Pendulums and their 

 phenomena are very easily projected by the shadow method ; 

 and so is the working of an Atwood machine, especially if 

 constructed with some reference to this use e.g. with a trans- 

 parent scale, thickly divided. By a little modification, how- 

 ever, the law of acceleration may be demonstrated in a very 

 superior manner. Let the descending weight consist either 

 of a vibrating metal fork (the sound of which is of no 

 consequence), or of a rigid pendulum, or of a piece of clock- 

 work ; either being furnished with a tracing point pressed 

 lightly against a long perpendicular strip of smoked glass. 

 The tracer should mark a bold line transversely across the 

 strip of glass. When now this tracer- weight is made to 

 descend by an additional weight as usual, its descent will cut 

 a series of wave-lines, nearly of the same amplitude as regards 

 width (quite so if clock-work be employed) ; but the lengths of 

 the waves will be greater and greater with the accelerated 

 downward motion, as will be seen upon the screen. The 

 whole series will project by the shadow method ; or measure- 

 ments may be taken, and different wave-lengths projected in 

 the slide-stage of the lantern. 



With a small Cavendish apparatus as constructed by Mr. 

 Vernon Boys, with quartz fibres, 1 the direct attraction of small 

 masses can be easily projected on the screen. 



112. Elasticity. By using a small spiral coil, traction can 

 be demonstrated ; or a larger coil may be used with the shadow 

 method. Torsion is projected by the vertical lantern or attach- 

 ment, using a divided glass dial for the scale. Flexure can be 

 projected in almost any way, and needs no description. 



113. Cohesion. This may be projected by the method of 



1 For full details see Proc. R.S., xlvi. 253. 



