THE MECHANICAL STAGE 



181 



enlarged so that a much wider preparation than the usual one inch 

 may be worked with great facility on this stage. The method of 

 attachment practically makes the mechanical stage one with the stage 

 of the microscope, as it is in contact with the fixed stage throughout 

 its entire length, and is clamped at the lower end to the top, and at 

 the upper end to the bottom of the stage. 



Both the rectangular movements are effected by rack and pinion, 

 the vertical one of which carries a bar (fixed as to horizontal movement) 

 against which the slide is pressed by a spring clip, and upon which 

 is mounted the rack and pinion for the horizontal movement ; the 

 end which presses upon the slip is tipped with cork in order to grip 

 the slide, and move it along the fixed bar ; when the milled head is 

 rotated, the slide actually rests on two small raised surfaces at either 

 end of the bar to minimise friction. This is without question a well- 

 made practical and use- 

 ful stage. Amongst 

 stages of this kind, how- 

 ever, the most original 

 and useful has been de- 

 vised by Mr. Nelson. 

 As seen in fig. 142, the 

 sliding bar has been 

 slotted and a movable 

 piece, which may be 

 called the shuttle, has 

 been fitted in the slot ; 

 this shuttle has a dia- 

 gonal rackwork at the 

 back, and a vertical 

 spiral pinion gears in it, 

 as is shown in fig. 143. 

 Above this pinion there 

 is a horizontal bevel 

 wheel which is geared 

 by friction to a vertical 

 wheel fixed on the usual 



FIG. 141. Baker's attachable stage (1898). 



horizontal pinion. The cock which holds, and is close to, the vertical 

 bevel wheel in fig. 143 is slotted underneath, a capstan-headed screw 

 (not shown in the figure) is fitted for the purpose of compressing this 

 spring part ; the amount of friction between the copper bevel wheels 

 can therefore be regulated at will. This capstan-headed screw is 

 placed some distance from the bearing, so that the length of the bar 

 between it and the bearing may form a stiff spring ; this renders the 

 motion equable. It will be noticed, therefore, that the transverse 

 movement is confined to the sliding bar. This sliding bar can be 

 removed so as to leave the stage perfectly plain. The heads of the 

 pinions which control the vertical movement have been kept below 

 the level of the stage so as to be out of the way of culture plates. 

 Three and a half inches of transverse movement is given to this 

 stage, and the manner of the holding the clip is quite new and 

 eminently serviceable. On the shuttle there are two sliding pieces, 



