MICROSCOPICAL EXAMINATION. 169 



thickness. Very thin ones were formerly of service when attempt- 

 ing difficult resolution with extremely oblique mirror illumination. 

 With a substage condenser, however, extreme thinness of the slide 

 is not only unnecessary, but undesirable, especially in high-power 

 work. The majority of modern microscopes that are pretended to 

 have any degree of excellence are provided with substage conden- 

 sers, either N.A. i achromatic or N.A. i*2o, or 1-40 Abbe. 



Now, these apertures are possible only when there is a layer of 

 cedar oil between the condenser and the slide. The principal 

 microscope makers list their condensers as having the apertures 

 mentioned, but not one of them, so far as he is aware, has the 

 honesty to state that these apertures exist only when they are used 

 with oil immersion, and that when used dry, as is usually the case, 

 the numerical aperture is very much less. If, now, the observer 

 desires to employ an immersion objective of high aperture and to 

 work it at its best, he must put oil on the condenser and focus it 

 for critical illumination. If the slide is an exceedingly thin one, 

 in bringing up the condenser to keep the oil in position, he will 

 project the flame image above the plane of the object under 

 examination. If the condenser is now depressed so as to make 

 the flame image coincide with the object, the oil is apt to run out, 

 especially if the microscope is incUned. The condensers are con- 

 structed to work with slides of medium thickness, and such slides 

 are the only ones that should be used. 



Cover-glasses should be selected with great care. Square ones 

 should never be used in the preparation of blood films, because it 

 is exceedingly difficult to obtain a good smear, and it is almost 

 impossible to mount them in a satisfactory manner for permanent 

 preparation. 



The most convenient size of round glass will be either three- 

 quarters of an inch or eighteen millimetres. American dealers 

 supply the covers in four grades, according to thickness — namely, 

 Nos. o, I, 2, and 3. The first two are altogether too thin for 

 general use, and should not be used under any consideration. A 

 great deal of blood work can be done better with dry lenses, and 

 the non-adjusting dry lenses in common use are corrected by their 

 makers for a certain definite thickness of cover-glasses ; and if a 

 thinner one is used, the image obtained will be imperfect. The 



