ZOOLOGY AND BOTA^^:, inCROSCOPT, ETC. 



945 



it — the perpendicular thickness of the working part of the stage, 

 i. e. from the surface of the object-plate to the under surface of the 

 ring which limits the free admission of oblique light, laterally or in 

 front, is hardly more than one-eighth of an inch, and yet the rigidity 

 is greater than in the previous model, where the thickness was about 

 half an inch. 



(2) As stated above, the rotating plate C, formerly rested on, and 

 projected slightly beyond, the sui-face of the stage-ring H ; it is now 

 countersunk to the thinnest level of the stage-ring, and the gradua- 

 tions are advantageously made on the surface, where they can be 

 more readily inspected. Small knobs are applied on the surface, 

 near the edge, for convenience of turning by the hand. 



(3) In the former model, the stage motions were obtained by means 

 of two plates of German silver, each about l-50th inch in thickness ; 

 the new model dispenses with the lower plate altogether (as first 

 suggested by Mr. Wenham), the plate F, traversing horizontally in 

 the box-fitting D, by means of a rackwork at its lower edge, and 

 acted upon by the toothed pinion of the milled head B, whilst the 

 vertical motion is given by the toothed pinion of the milled head A, 

 gearing into a vertical slot rackwork in the rotating plate C, — the 

 box D, with the pinions, and the plate F, moving together, all carried 

 on a dove-tail plate sliding in a corresponding slot cut in the rotating 

 plate, certain guide-pieces (like the peg shown behind D) serving to 

 prevent lateral rocking. 



(4) In the earlier model the rectangular motions were each con- 

 trolled by a pinion and milled head on separate axes : now they are 

 combined upon the same axis. The milled head A, attached to a 

 solid pinion passing through 

 the box D, is held beneath the 

 rotating plate by a screw and 

 friction-washer, and gives ver- 

 tical motion, as explained 

 under (3) ; the milled head B 

 is attached to a hollow pinion 

 (encircling the solid one) fitting 

 in a ring fixed within D, and 

 gives horizontal motion to the 

 plate F, as noted under (3) ; 

 the fittings of these pinions are 

 so arranged that the motions 

 are quite independent. In 

 this last improvement (as ap- 

 plied to this form of stage) Mr. 

 Tolles has been preceded by 

 Mr. Wenham and Messrs. 

 Watson. 



The mechanism of the stage-movements will be understood by refer- 

 ence to the section drawings (Figs. 222 and 223). In Fig. 222, A is a 

 milled head attached to the inner arbor, communicating motion to the 

 toothed pinion a, which works in the rack G attached to the revolving 



Ser. 2.— Vol. I. 3 K ° 



Fig. 222. 



