USE OF THE COMPOUND MICROSCOPE 33 



to bring the edge of the diaphragm on tlie source into 

 view, and this is focused by the condenser. Then, the part 

 of the object to be studied having been found, a small 

 diaphragm is put on the source (for the high powers, 

 usually a 3-millimeter diaphragm). The image of this 

 diaphragm is placed in the center of the field over the 

 portion of the object to be examined. Then the high-power 

 oil-immersion objective is focused down until there is oil 

 contact. It is then raised slightly and lowered to focus 

 on the edge of the small image of the diaphragm, which 

 will be readily ^dsible. The object is easily found in 

 the circle of light. The approach to the focal point is 

 shown by a rapid increase of light. In this way of focusing 

 it is seldom that the high-power water or oil-immersion 

 objective comes into contact with the cover-glass. (A 

 reflecting prism is preferable to a plane mirror since it 

 gives a single image of the diaphragm, which is easy to 

 focus.) 



Objectives. — Dry medium and dry high objectives are 

 best used on objects in air. For objects in any liquid, 

 they require adjustment by a correction collar (or by 

 altering the tube length) ; and the same is the case if they 

 are employed with cover-glasses of the wrong thickness. 

 The cover-glasses used should be slightly under 0.17 

 millimeter, with objects in water or balsam. Hence one 

 might think dry objectives would be useful, if specially 

 corrected by the optical firm making them, for employment 

 ivithout cover-glasses. Such objectives are now made for 

 use in metallurgical work. They can also be utilized in 

 ordinary work, as Coles (46) has shown with regard to the 

 stained smears used in pathological laboratories. Also, 

 for metallurgical work, medium-power objectives are 

 sometimes made to be used not dry, but as oil immersion. 

 These might also be useful in laboratories where the high- 

 power oil-immersion objectives are usually employed with- 

 out cover-glasses. (No doubt, even serial sections could 

 readily be mounted so as to be conveniently \'iewed with- 

 out covers.) 



