2 APPARATUS. 



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 w r ithin the ocular. When using a disk micrometer, it is 

 supported by this diaphragm, and the outlines of the image are cut by 

 the rulings on the glass disk, and so we are enabled to measure the size 

 of the object being 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 important 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 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 focus an object when 

 a high-power dry objective (1/6 in. or 1/8 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 a No. 2, but with 

 a little practice in cleaning cover-glasses this is negligible. 



One of the most fruitful causes of the crushing of microscopical 

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



