126 Prof. J. A. E wing. On Measurements of small [May 16, 



exceeding the range of the eye-piece scale, Q is brought back into the 

 field of view by turning P through a distance which is observed by 

 means of a divided head. 



FIG. 2. 



Fig. 2 is a view of the complete instrument, taken from a photo- 

 graph. The clips B and C are in this instance set at 8 inches apart, 

 and B is jointed to B' by a transverse joint, giving P freedom to 

 accommodate itself to a hole in C. The joint between B and B 

 consists of two upright pins fixed in B, one of which presses up into 

 a hole and the other into a slot in B', the line of this hole and slot 

 being perpendicular to the axis of the set-screws by which the clip is 

 attached to the rod under test. Hence, so far as movement about 

 the axis of the set-screws is concerned, B and B' act as a rigid 

 whole. This movement is prevented by the gearing of P in the hole 

 in the lower clip C. The piece B' is a frame consisting of two 



