146 



THE AMERICAN MONTHLY 



[August, 



to the sector. Wi h the sector cLiniped 

 at the usual angle, this latter arrange- 

 ment furnishes very ready and practi- 

 cal means of varying the illumination 

 in altitude from 0° to nearly 90°, either 

 direct from the mirror or combined 

 with a low-power condenser, the hem- 

 ispherical or Wenham's semi-disc illu- 

 minator. 



3. The rotation of the whole 

 instrument on the lower base-plate 

 on an axis which is the prolonga- 

 tion of the optic axis when the in- 

 strument is vertical, as shown 

 in the figure. 



4. The swinging of the tail- 

 piece suspended on an axis, the 

 centre line of which passes 

 through the object on the stage, 

 cutting the optic axis at right 

 angles. 



5. The complete rotation of the 

 mechanical stage upon the optic 

 axis, secured by the milled-head.s 

 for the rectangular motions being 

 placed on a vertical axis on the 

 stage and acting entirely within 

 the circumference. This stage 

 can be easily removed, and may 

 be replaced by a glass or other 

 stage. 



6. The complete rotation of 

 the mechanical sub-stage upon 

 the optic axis. 



7. The partial rotation of the lamp, 

 etc., in azimuth, upon an arm pivoted 

 in the centre of the lower base-plate. 



The Coarse Adjustment is on the 

 Jackson principal, and is worked by 

 "diagonal rack and spiral pinion. 



An entirely new construction of 

 fine adjustment is applied to this mi- 

 croscope, consisting of a V-slide acted 

 upon by two '' snail "-cams, between 

 the edges of which revolves a steel 

 roller, forming the axis of and actuated 

 by a large milled-head passing longi- 

 tudinally through the slide of the 

 coarse adjustment, and projecting 

 slightly on either side in a convenient 

 position for work. The V-slide is 

 fitted within the body-tube, carries af 

 its lower end the nose-piece, and is 

 pressed downward by a spiral spring, 



against which it is moved by the rev- 

 olution of the cams. By this system 

 an extremely sensitive and direct 

 focussing is obtained, which will 

 compare favorably with any hitherto 

 applied to the Jackson model of mi- 

 croscope." 



A thin, diatom-stage is provided, 



Fig. 25. 



represented in fig. 25, with mechan- 

 ical lectangular motions on one axis, 

 entire rotation by milled-head beneath 



Fig. 26. 



