USE OF THE MICROSCOPE. 51 



English and American microscopists prefer an inclined 

 microscope, while the German and French instruments 

 being usually vertical do not permit this arrangement. 



Adjustment. The details of microscopic adjustments 

 are only to be learned by practice, yet a few directions 

 may be instructive. The selection of the objectives and 

 eye-pieces depends on the character of the object. As a 

 general rule, the lowest powers which will exhibit an 

 object are the best. It is best to use weak eye-pieces with 

 the stronger objectives, yet much depends on the perfec- 

 tion of the glasses employed. 



The focal adjustment can be made with the coarse ad- 

 justment or quick motion when low powers are employed ; 

 but for higher powers the line adjustment screw is essen- 

 tial. Care must be taken not to bring the objective into 

 close or sudden contact with the thin glass cover over the 

 object, and, in changing object-glasses, the microscope 

 body should be raised from the stage by the coarse adjust- 

 ment. 



The actual distance between the object and object-glass 

 is much less than the nominal focal length, so that the 

 1 inch objective has a working distance of about J an 

 inch, the Jth of about ^th of an inch, while shorter ob- 

 jectives require the object to be covered with the thinnest 



Sometimes, in high powers, and especially with immer- 

 sion-lenses, an adjustment of the object-glass is necessary 

 in order to suit the thickness of the glass cover. With 

 thick covers the individual lenses must be brought nearer 

 to each other, and, with very thin covers, moved farther 

 apart. 



If immersion-objectives be employed a drop of water is 

 placed on the glass cover with a glass rod or camel's-hair 

 pencil, and a second drop on the lens. The lens and object 

 are then approximated till the drops flow together and the 

 focus is adjusted. By turning the ecrew of the objective 



