274 Transactions of the Society. 



tube, and lie inside the two small tubes which are attached to the 

 back of that tube. The upper one of these small tubes has already 

 been mentioned as the guide which prevents rotation of the coarse- 

 adjustment tube. The lower end of the wire is fixed rigidly, the 

 upper through a spring held in tension. At about its middle point 

 the wire takes one turn round the cylindrical barrel. The barrel 

 has its bearings in a ring (L, fig. 51), which embraces the large fine- 

 adjustment tube. To the outer ends of the barrel are screwed the 

 milled heads of the coarse adjustment. The friction of the wire 

 upon the barrel is sufficient to cause the coarse-adjustment tube to 

 move up or down when the barrel is rotated. The reasons for 

 adopting this device in preference to the usual rack-and-pinion are 

 two : first, the relatively small cost of manufacture, and, secondly, 

 the fact that its action upon the tube is only a direct pull in the 

 direction of the desired movement. It exerts no side thrust, such 

 as is caused by a rack-and-pinion. The absence of teeth causes the 

 motion to be extremely smooth and regular. The wire is made of 

 hardened steel, silver-plated, and is protected, when the instrument 

 is put together, by the small tubes in which it lies. 



The four guides in the limb, in which the fine-adjustment tube 

 slides, have been already described. It remains only to deal with 

 the means by which the tube is held against those guides, and 

 prevented from rotating about its long axis. Both these ends are 

 secured by means of a rod (M, fig. 51), hinged to the ring which 

 surrounds the fine-adjustment tube, and passing backwards through 

 a hole in the back part of the limb. A spring (N, fig. 51), held in 

 compression between the limb and a nut (0, figs. 51, 53) screwed 

 on to the end of the rod, pulls the fine-adjustment tube against its 

 four guides. The hinged joint, whose axis is horizontal, allows the 

 fine-adjustment tube to move up and down through a small distance, 

 moving the rod in or out of the hole in the limb as it moves ; at 

 the same time it does not allow rotation of the tube against its 

 long axis. 



Fig. 50. — Section through Upper Part of Limb and Body Tubes. 



C, ring carrying guides of coarse-adjustment tube. 

 Other letters as in fisr. 49. 



There is a spring (P, fig. 51) in tension between the upper part 

 of the fine-adjustment tube and the more remote end of the rod. 

 This spring performs two functions. In the first place, it ensures 



