164 ELEMENTARY CHEMICAL MICROSCOPY 



comment from workers with the microscope. All efforts to obtain 

 this lamp by the author have thus far failed. Its description 

 and applications cannot therefore be given. 



Nosepieces. Objective Changers. — In ordinary microscopic 

 investigations frequent changes from one objective to another 

 in order to obtain increased magnification are usually necessary. 

 To avoid the annoyance and loss of time required to unscrew 

 one objective and reinsert another, various devices have been 

 suggested. Those almost universally employed by biologists 

 are known as revolving nosepieces and are shown in Figs. 98 

 and 99. The illustrations show their construction and opera- 

 tion sufficiently well to need little comment. The nosepiece is 

 attached to the body tube of the microscope. It may accom- 



Fig. 98. Revolving Nosepiece for Fig. 99 Dust-proof Revolving 



Three Objectives. Nosepiece. 



modate two, three or four objectives as the case may be. The 

 better type is shown in Fig. 99. It is circular and almost dust- 

 proof, while in the type shown in Fig. 98, if by chance the 

 objectives are not turned under the shields dust falls upon the 

 back lens combinations. Owing to the almost impossibility of 

 constructing these nosepieces so that each objective will be 

 properly centered when turned in place, many investigators pre- 

 fer objective "holders" or "changers'" instead of revolving 

 nosepieces. Three forms of objective changers are illustrated 

 in Figs. 100, 101 and 102. In the case of those of the form of 

 Figs. 100 and 101 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 £eiss device, Fig. 102, the objective is screwed 

 into the sliding block b and is pushed into the slides in the plate 



