C86 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



box L outside the lower end of the body-tube. The arm C is moved 

 against the spring by the differential-screw mechanism (with milled 

 head D) which is gimballed on a bracket E attached to the upper 

 part of tho body-tube. 



The differential-screw mechanism consists of a steel rod F 

 (connected with the milled head D) which has two screw threads at 

 the lower end, one working in a thread cut in the end of the inner 

 tube G, and the other in the block 11, which is soldered within the 

 sheath J). When the milled head is turned to the left, the block, 

 and with it the sheath, moves downwards while the rod itself, carry- 

 ing the block and sheath, moves upwards. As the screws are cut 

 respectively to 45 and 52 threads to the inch, the resultant motion is 

 equivalent to the difference between the two screws, that is, to the 

 motion of a screw of nearly 335 threads to the inch. 



The end of the sheath is tipped with a small sphere K of polished 

 steel, while the projecting arm of the nose-piece tube against which 

 the end works has a corresponding concave bed of polished agate. 



Delicate Fine Adjustment.* — A delicate system of fine adjustment 

 is described anonymously, but said to be " after Dr. Eoyston-Pigott." 

 It is shown in section and plan in fig. 142. 



The primary wheel carries an axis of steel, 1-3 inch thick, 

 having an external thread exactly 101^ turns per inch, which travels 



Fig. 142. 



in a brass nut having 60 turns of a corresponding thread. This 

 wheel has 100 teeth on the rim and engages a pinion of 10 teeth 

 which forms one piece with the secondary wheel (removable at will), 

 also divided into 100 parts. Each of the divisions on the secondary 

 wheel represents a movement of the focal plane through a space of 

 1/230,000 in. There is also provision for changing the fulcrum so 



* Eng. Mecb., xliii. (1886) p. 340 (2 figs.). 



