138 



ELEMENTARY CHEMICAL MICROSCOPY 



properly centered when turned in place, many investigators pre- 

 fer "objective holders" or "changers" instead of revolving nose- 

 pieces. Three forms of objective changers are illustrated in 

 Figs. 85, 86 and 87. In the case of those of the form of Figs. 85 



FIG. '85. Bausch & Lomb Clutch 

 Objective Changer. 



FIG. 86. Leitz Clutch Objective 

 Changer. 



and 86 a flanged collar is attached to each objective. Pressing 

 the levers together opens the clutch, and permits the objective 

 with collar attached to be pushed in place. Upon releasing the 

 levers the objective is seated and securely 

 held. In the case of the Zeiss device, 

 Fig. 87, the objective is screwed into the 

 sliding block b and is pushed into the 

 slides in the plate a which is attached 

 to the end of the body tube of the mi- 

 croscope. The screws S, S', turned by 

 a small key, permit the accurate center- 

 ing of each objective. This is the best 

 type of device when centering is essen- 

 tial, but requires a special box for holding 

 the objectives to which the blocks b have been attached. With 

 the clutch or clamp type (Figs. 85 and 86) the ring is of such 

 diameter as to permit placing the objectives in their usual brass 

 boxes. 



Sedimentation Glasses. The apparatus illustrated in Fig. 88, 

 commonly known as Spaeth's sedimentation glass will be found 

 a most useful laboratory device. The liquid containing the sedi- 

 ment to be examined is poured into the glass with its stopcock 



FIG. 87. Zeiss Centering 

 Objective Changer. 



