SLIDE-CENTERER MEASURING WEDGE. 2?$ 



which is simply a piece of very stiff card, upon which the 

 lines and circles can be drawn. 



Another form of centerer for slides was illustrated in 

 ' Science-Gossip ' for 

 1879, from which the 

 accompanying sketch 

 is taken (Fig. 225) ; 

 the device is so sim- 

 ple that it needs no 



description. 



FIG. 225. 



The object is nearly always covered with thin glass, 

 either squares or circles, and is sold by the ounce in 

 sizes for circles, increasing by eighths of an inch. It is of 

 importance that for high powers used dry the cover-glass 

 should be of uniform and great thinness. When pur- 

 chased they should be selected according to thickness, and 

 each variety used for its special purpose. The thickness 

 may be ascertained in various ways. In one, the cover 

 may be held in the stage forceps edgeways, and measured 

 by observation with the micrometer eye-piece shown at 

 Fig. 202. 



Another method is by the lever of contact as made by 

 Messrs. Ross and Co., or the cover-glass measurer devised 

 by M. Schonemann, called the "measuring wedge," and 

 shown in Figs. 226 and 227. 



FJG. 226. FIG. 227. 



In order to measure the thickness of a cover-glass the 

 instrument should be free from dust, and the wedge drawn 

 back until the cover can be placed between the wedge and 



T 2 



