TURNTABLES. 



277 



springs for the purpose of holding down the glass slip ; it has 

 been the parent of several forms supposed to be of great 

 benefit to the microscopist. In order that the slides may 

 be easily centered, the table is usually engraved with a 

 series of circles ; but it is readily seen that such centering 

 can only be approximate. 



In order to centre the slides accurately in one direction, 

 that of width, Zentmayer introduced the simple device of 

 fixing a couple of pins equidistant from the centre and at 

 opposite sides of the table, the slide being so arranged that 

 it touched both of these pins. This centres for the width, 

 and in length this is accomplished by a series of circles near 

 the edge of the table, the operator making the adjustment 

 from inspection of these. 



In 1870 Dr. Matthews devised a turntable (Fig. 228) to 

 accurately centre slides in the direction of their width, and 

 this possessed the further advantage that no springs or 

 other portions of the table rose above the slide, to catch 

 the fingers or brush, during its revolutions ; and in the next 

 year Mr. J. B. Spencer, in a 

 communication to 'Science- 

 Gossip,' showed how this 

 might be made in hard wood 

 by the microscopist himself. 



In ' Science - Gossip ' for 

 1874 Mr. Bridgman described 

 and illustrated a form of turn- 

 table (Fig. 229) which, though 

 not self-centering, enabled a 

 slide to be always placed in 

 the same position upon it, so 

 far as the centre of rotation went. In 1875 Mr. C. F. Cox, 

 of New York, devised a self-centering turntable, con- 

 sisting of the circular revolving plate, in which was cut a 

 slot in the direction of its diameter, and in this were 



FIG. 229. 



