OBJECTIVES 3 



which is meant an ocular in which the lower lens (collective) assists in forming the real 

 inverted image which is focused at the level of the diaphragm within the ocular. 

 When using a disc micrometer, it is supported by this diaphragm, and the outlines of 

 the image are cut by the rulings on the glass disc, and so we are enabled to measure 

 the size of the object examined. The measurement of various bacteria, blood-cells, 

 and parasites is exceedingly simple and assists greatly in the study of bacteria, and is 

 indispensable in work in animal parasitology. (For details of micrometry see section 

 on blood- work.) When an ocular is termed positive, it refers to an ocular which acts 

 as a simple microscope in magnifying the image, the image being formed entirely by 

 the objective and located below the ocular. By fixing one end of a hair on the rim of 

 the diaphragm inside the ocular with a minute drop of balsam one has a satisfactory 

 pointer to locate any particular cell in the microscopic field. 



Objectives are usually designated by their equivalent focal distance. It is impor- 

 tant to remember that the equivalent focal distance does not represent the working 

 distance of an objective, by which is meant the distance from the upper surface of 

 the cover-glass to the lower surface of the objective. Thus a K-inch objective may 

 have to be approached to the object so that the distance intervening may be only 

 % inch or even less. This explains the frequent inability to focus an object when a 

 high-power dry objective (^-inch or ^-inch) is used with a rather thick cover-glass 

 the objective possibly having a short working distance, so that the thickness of 

 the cover-glass does not allow of any free working distance. 



Instrument makers generally specify the thickness of cover-glass to be used with 

 a certain tube length, but as a practical matter it will be found convenient to use No . 

 i (very thin) cover-glasses. The principal objection to these is that they are more 

 fragile than the No. 2, but with a little practice in cleaning cover-glasses this is 

 negligible. Immersion lenses are less affected than dry lenses by the question of a 

 certain thickness of cover-glasses for a certain tube length. 



One of the most fruitful causes of the crushing of microscopical ob- 

 jects and the overlying cover-glass or, what is far more important, the 

 breaking of the cover-glass of a hanging-drop preparation and conse- 

 quent risk of infection is the attempt to focus with the fine adjustment. 



It should be an invariable rule for the worker to bring his objective practically into 

 contact with the upper surface of the cover-glass, then using the coarse adjustment 

 (rack and pinion) to slowly elevate the objective, looking through the eye-piece at 

 the same time. In other words, obtain focus with the coarse adjustment and main- 

 tain it with the fine adjustment (micrometer screw). The fine adjustment should 

 only be used after the focus is obtained. 



Oil Immersion. In using the oil-immersion objective always dip the 

 lens in the oil and practically touch the cover-glass the eye being at 

 a level with the stage before beginning to focus. With the coarse ad- 

 justment one can feel the contact with the cover-glass, which is impos- 

 sible with the fine adjustment. It saves time and disappointment to 

 make a preliminary examination of a preparation requiring the high 



