THE MECHANICAL STAGE 



183 



and these hold the slip by the two lower corners, as seen in fig. 142 ; 

 and this mode of gripping allows for the employment of the in- 

 valuable method of touch on the edge of the slide for discovering 

 working distance and focus. A plain sliding bar may be substituted 

 for the mechanical bar ; this forms a semi-mechanical stage as shown 

 in fig. 144. The mechanical movement being only imparted to the 

 lugs at the side of the stage, the bar may be moved by the hand by 

 sliding as in an ordinary plain stage without the employment of the 

 mechanical movement. 



The stage is of aluminium, and its size is 4^ x 7 inches. 



Another attachable stage having many advantages is made by 

 Reichert and shown by fig. 145. It can be used with any instrument 

 of the Continental type, is very carefully made, and the scales 



FIG. 145. Beichert's attachable stage. (About half natural size.) (1892.) 



attached are divided to read by means of a vernier to O10 mm., and 

 the range of movement is an inch in both directions. 



An attachable mechanical stage is also made by the Bausch and 

 Lomb Optical Company of Rochester, New York, having great 

 merit and some special points ; and this firm is in advance of all 

 other makers that we know of in making an attachable revolving 

 mechanical stage. 



There is much similarity to the American mechanical stage in one 

 made by Carl Zeiss and illustrated in fig. 1 46 . Of course the principle, 

 as primarily in all the others, is that suggested by the late Mr. Mayall, 

 and afterwards by Reichert. Two sliding pieces, mounted at right 

 angles to one another, are moved by means of two milled heads, S, T. 

 They pass along millimetre scales which serve to record any particular 

 position. 



The demand for these attachable stages is, we presume, consider- 



