2 APPARATUS 



for urine and blood counting, with a i/8-in. for examining hanging-drop 

 preparations and for quick examination of blood smears). An apo- 

 chromatic objective costs about three times as much as an achromatic 

 one and, except in photographic work, has little if any advantage. 



As regards oculars (eye-pieces) a No. 2 and a No. 4 will best meet 

 the requirements. For high magnification a No. 8 may be of service. 

 The Zeiss oculars are numbered according to the amount they increase 

 the magnification given by the objective; thus a No. 2 increases the 

 magnification, given by the objective alone, twice; a No. 8, eight times. 

 Some oculars are classified according to the equivalent focal distance, 

 and are referred to as i/2-in., i-in., and 2-in. oculars. 



The oculars in common use are known as negative oculars, by 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 being located below the ocular. 



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

 portant to remember that the equivalent focal distance does not represent the work- 

 ing 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 i/4-in. objective 

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

 only 1/6 in. or even less. This explains the frequent inability to locus an object 

 when a high-power dry objective (i/6-in. or i/8-in.) 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 

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

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

 consequent risk of infection is the attempt to focus with the fine adjust- 

 ment. It should be an invariable rule for the worker to bring his object- 



