524 Mr. E. G. Coker on Instruments for Measuring 



employing a reading-microscope to observe the relative angular 

 displacement of a radial line upon the vernier-plate. The 

 edge of a thick wire is a very convenient line for observation, 

 and has been used with notable success in an extensometer 

 designed by Professor Ewing (Proc. Roy. Soc. May 1895). 

 The instrument is shown by fig. 2, in which A is the graduated 



' Fig. 2. 



w p 







R Q 



circle mounted upon a chuck, B, and furnished with a vernier- 

 plate, J, an arm, 0, of which carries a wire, P. 



A reading-microscope is carried in the sleeve, R, of an 

 arm, !3, mounted upon the short cylinder, C, which latter is 

 gripped upon the test-bar by screws. 



The reading-microscope has an eyepiece, T, provided with 

 a glass scale, and a right-angled prism, U, is interposed 

 between this and the objective, W, so that readings can be 

 easily taken. The tube, Q, is free to slide or rotate in its 

 guide, R ; but in order to readily focus the wire this latter is 

 carried in a frame, X, pivoted on the vernier-plate, J, and 

 adjusted by a screw. 



The microscope arm, S, is secured to the cylinder, C, by a 

 divided collar, the two halves of which are pivoted on one 

 side and the free ends are clamped by screws. If it is desi- 

 rable that the telescope be turned round or released altogether, 



