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PROFESSOK W. E. AYRTON, MR. T. MATHER AND MR. F. E. SMITH: 



groove cut in a brass bar carrying a plane mirror M ; the bar is free to rotate about a 

 steel axle, its movement being in a vertical plane. A comparatively strong brass 

 spring on each side of the bar causes it to continually press against the piston if the 

 rounded end of the latter projects beyond the stop S. The vertical distance between 



1 



Fig. 11. Machine for measuring diameters of cylinders. 



the axis of the mirror and the contact point of piston and brass bar is about 1*5 millims., 

 and the height of the micrometer head above the upper surface of the girder is 

 30 centims. When the barrel B of the micrometer advances, the piston, mirror, 

 bearings, &c., advance with it until the plane end of the piston comes in contact with 

 a rigid body ; an advance of the micrometer barrel then results in a tilt of the mirror 



Fig. 12. Optical lever for measuring diameters of cylinders. 



nd the consequent deflection of a spot of light. All the parts are well made and the 

 bearings lubricated with clock oil. A Nernst filament was used as a source of light, 

 and a lens produced a sharp linear image on a white surface 150 centims. distant from 

 the mirror. A forward motion of the micrometer barrel of I/A resulted in a deflection 



