128 Prof. J. A. EAving. On Measurements of small [May 16 S 



Fia. 3. 



often necessary condition in the application of the instrument to such 

 pieces as the girder flanges. The clips are (as in Professor Onwin's 

 extensometer) adjustable to wide or narrow bars. The pieces B and C 

 engage by means of a screw-point in a hole, the necessary pressure 

 being supplied by springs, with the effect that the apparatus can be 

 used in any position, horizontal, vertical, or inclined (as in the figure) . 

 The sighted wire Q, is on a prolongation of C beyond P, giving a 

 mechanical multiplication of three to one. Between the objective 

 and the eye-piece of the microscope is the box N", containing a series 

 of two totally reflecting prisms, which turn the line of sight through 

 two right angles. This arrangement keeps all parts of the instru- 

 ment compactly together on one side of the bar or plate under test. 

 A clamp is provided at J which, by the tightening up of a single nut, 

 disengages P from its hole and fixes the two portions, B and C, in 

 relation to one another, so that the instrument may then be handled 

 as a rigid whole, and be readily applied to the bar which is to be tested, 

 the axes of the clips being then parallel and at the proper distance 



