Variably Coupled Vibrations. 339 



for focussing a spot of light therefrom on a photographic 

 plate, B, placed sensitive side uppermost in a frame which 

 can be moved uniformly in a direction perpendicular to the 

 motion of the pendulums. The experiment is performed in 

 a dark room and the plate developed immediately. 



Fur. 3. 



The elastic pendulum consists of an oak lath clamped in a 

 vice at A and carrying a bob of special construction at its 

 upper end. The details of the latter can be seen from fig. 4, 

 which is shown partly in section. The end of the lath S is 

 fitted with a brass cap carrying a screw T. This screw 

 serves for the purpose both of clamping the lead weights R 

 between washers, as shown, and of forming the central 



