ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



257 



thus avoiding any back-lash. The worm-wheel is carried in the invar 

 frame K, pivoting; on pin P, which frame is fitted with a slot to accom- 

 modate the rise and fall of the worm-wheel, so that worm, wheel and frame 

 can move together. The worm or tangent-wheel W, as shown in the 

 end elevation (fig. 55), is turned solid with its shaft, and is rotated by- 

 means of the aluminium thumb-screw T, which is screwed to the worm- 

 wheel shaft, as shown at fig. 55. The invar worm-wheel is kept in gear 

 with the phosphor bronze worm by means of the constant pressure of 

 the tuning-fork-shaped spring E, which spring is screwed to the hori- 

 zontal lifting-bar H by a couple of screws, as shown by the elevation 

 at fig. 53. The worm-wheel X is driven fast on the centre spindle, 



>" 



«v BMUMto 'JRC. 



Fig. 55. 



and further secured by the No. 10 B.A. screw (fig. 53). The lifting- 

 bar Hj is fitted with a balance weight, and both lifting-bars have ivory 

 tips. The balance-weight was carefully reduced from an excess until 

 exact balance was obtained, this being highly necessary ; balance in the 

 other direction being similarly attained by the bearing Q (fig. 54). 

 The pointed ends of the conical feet F have the sharpness re- 

 moved sufficiently to prevent them scratching the lens. The vertical 

 spindle is screwed 100 to the inch, the worm-wheel has 500 teeth, 

 one-fiftieth inch pitch, and the worm has 50 threads per inch ; thus 

 for one revolution of the worm the vertical spindle, together with the 

 worm-wheel, is raised or lowered one fifty-thousandth of an inch, or 

 one-millionth of an inch for one-twentieth of a revolution of the 

 worm-arbor. It was found that the error of this spherometer was 



