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Mr. L. 0. Jackson on 



guides perpendicularly to the motion of M, the room in which 

 experiment is made being dark. The box part of: M contains 



an electric battery which lights a small lamp inside C. The 

 light then passes through a pinhole and is focussed in B by 

 a lens in D. By this means a trace is obtained directly for 

 the motion of M. If the apparatus is to be used for demon- 

 stration purposes for a class of students, the bob M is replaced 

 by a funnel and sand, as in arrangements used in papers 

 previously mentioned. 



