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Mr. L. C. Jackson 



contact of the small lamp-holder G. The light from the 

 lamp U passes through the pinhole V and the lens W, 

 thereby allowing a spot of light to be focussed on the photo 

 plate by means of the mirrors JD and the lens L (fig. 3). 

 The mirrors D can be adjusted to any angle with one 

 another, and can be clamped in such a position on a hori- 

 zontal slide that they occupy the correct position relative to 

 N, the latter movement being necessary, as the equilibrium 

 position of N varies with the distance of the point of sus- 

 pension of the gravity pendulum M from the base of the lath. 



Fkr. 4. 



The gravity pendulum consists of a steel rod which can 

 slide in a steel tube carrying a cylindrical lead bob which 

 supports the optical arrangements. These latter are exactly 

 similar to those of the bob N. The gravity pendulum swings 

 on a knife edge attached to the rod P, which screws into the 

 lath at various distances along its axis. 



The electrical connexions are as follows : — A wire is 

 fastened to each side of the lath throughout its whole length, 

 while a third wire (GrH in fig. 4) runs down the back of the 



