130 Messrs. Morrow and Watkin on an Interference 



considered it advisable to find out how far the extension 

 readings were reliable. 



For these purposes it was decided to calibrate the instru- 

 ments by interference methods; and accordingly, after some 

 preliminary trials, the apparatus described in this paper was 

 designed. The following points were kept in view as being- 

 essential : — (1) That the calibrator should be as far as possible 

 self-contained and easily made ready for use ; (2) that any 

 displacement accidental or otherwise should affect both the 

 interference apparatus and that being calibrated simul- 

 taneously and equally ; (3) that it should be unaffected by 

 external disturbances ; and (4) that there should be no 

 error possible in the determination of its own displacement. 



In testing strain-meters by comparison with reading- 

 microscopes or comparators there is great difficulty, owing to 

 the unavoidable backlash of the screw, in ascertaining 

 whether the instrument is itself free from a similar defect. 

 The apparatus here described is not open to this objection. 



Description of the Apparatus. 



In figures 1 & 2, ff is a rigid cast-iron frame into the 

 top of which is screwed a steel tube a. The lower portion 

 forms guides g g, between which slides a gun-metal sleeve c. 

 The steel tube b is attached to c by a set-screw, and the 

 whole can be raised or lowered by the fine- threaded screw s 

 actuated by the levers I V. 



The extensometer to be tested is attached by means of its 

 gauge screws to a and b as to an ordinary test-piece. The 

 displacement to be measured is therefore that of b relatively 

 to a, and this is effected as follows : — 



To the lower end of a, and at right angles to its axis, is 

 attached a thin piece of optically plane glass p, whilst b 

 carries at its upper extremity nearly in contact with p a lens 

 q of black glass and small curvature. This lens is supported 

 by the levelling table r, by means of which its centre of 

 curvature is placed accurately on the axis of the instrument. 



Light o£ known wave-length passing through a hole in a 

 is directed to this optical system by a piece of plane glass m, 

 set at an angle of 45°; and the interference-rings thus pro- 

 duced are viewed through the microscope k which slides in 

 the upper tube. 



When the screw s is turned b is raised or lowered through 

 a small distance, and a ring appears or disappears for every 

 half wave-length of light. 



In the eyepiece of the microscope are three cross-wires, 



