OF THE MICROSCOPE. 211 



and are almost always made by the microscopist himself by 

 means of a little instrument known as a turn-table or 'whirling 

 table, of which there are several different forms in market. A 

 cheap and efficient form is shown in Fig. 62. The table is sup- 

 ported by a spindle upon which it turns, motion being com- 

 municated by means of a milled ring. The slide is held in its 

 place by two spring clips, and it is brought to the centre by 

 means of a guide or bar, c, with a square projection. This is 

 carefully arranged, so that a slide 3 by 1 shall be accurately 

 centered. Hence it follows that the rings and cells on all the 

 slides put up by the owner may be instantly and accurately 



Fig. 62. TURN-TABLE. 



centered by simply placing them on the table and bringing 

 them up to a firm bearing against the guide. This bar or 

 guide may, however, be instantly removed when desired, and 

 when this is done, any cell may be truly centered by the usual 

 methods. This turn-table, therefore, enables us always to bring 

 cells of our own make instantly to a perfectly accurate centre, 

 while other cells can be centered at any time with very little 

 trouble. 



To most turn-tables there lies the objection that the devices 

 for centering and holding the slide make one side heavier than 

 the other, and consequently, as every mechanic knows, irregu- 

 lar and eccentric motion is the result. On many otherwise 

 well-made instruments it is, from this cause, impossible to make 

 $ true cell, particularly if we attempt to work at a. high speed, 



