GENEKAL MICKOSCOPY 



apertures than corresponding apochromats, 

 but greater depth of field, less curvature of 

 field, and a better lendition of contrast. 

 These factors make achromats easier to use. 

 There are other objectives which are of 

 iiitennediate correction, i.e., they are not as 

 well corrected as apochromats but better so 

 than ahcromats. These are called fluorite 

 objectives. Generally lower magnifications 

 do not require the employment of apochi'o- 

 mats; however, the latter must be used at 

 the highest magnification in order to attain 

 the ultimate resolution. Consequently, it is 

 often advisable to acquire a set of objectives 

 by which the lower magnifications are 

 achieved with achromats, intermediate mag- 

 nifications wtihout oil immersion, by fluo- 

 rites, and the ultimate resolution at the 

 highest magnification by an oil-immersion 

 apochromat. 



Objectives are classed also as to magnifica- 

 tion, working distance, focal length, and 

 numerical aperture. All of these are inter- 

 related. Generally, the longer the focal 

 length, the smaller is the magnification and 

 the numerical aperture, but the greater the 

 working distance. The exception is a special 

 type of long-w^orking distance objective of 

 high N.A., which has a front lens of un- 

 usually large diameter. Even in this case, 

 the depth of field is low. Some objectives 

 are especially designed for the observation of 

 particular samples. Some are optically cor- 

 rected for use on a sample covered by a 

 coverglass of a specified thickness ; others are 

 not corrected in this sense, and are to be 

 used on uncovered specimens. Some are 

 constructed so as to be strain-free and suit- 

 able for polarized-light studies, and yet 

 others are made to perform observations by 

 phase contrast, dark field, etc. It is of ut- 

 most importance that the potential worker 

 in microscopy be cognizant of the demands 

 to be placed upon the objectives, so that a 

 judicious selection may be made. Excellent 

 publications are available, which provide 

 detailed information on microscope optics 



(8). Newly computed lens formulas have 

 made it possible for several manufacturers to 

 place high-quality flat-field objectives on the 

 market. These are not mentioned in the 

 above-cited publications, but one should 

 look into their description by the manufac- 

 turer. They yield images which are in focus 

 over the entire field of view, making it pos- 

 sible to obtain photomicrographs of large 

 areas with objectives of high numerical 

 aperture. 



Eyepieces (oculars) are designed by the 

 makers to be used with their microscopes 

 and objectives. An eyepiece of one manu- 

 facturer should not be combined in a sy.stem 

 with, the objective of another unless suitable 

 corrections in tube lengths are possible. The 

 following chart illustrates the magnitude of 

 discrepancies which exist among microscope 

 tube lengths and ocular focal plane positions 

 supplied by various manufacturers. 



Several types of oculars are available, but 

 it does not fall within the scope of this article 

 to describe them in detail. However, it 

 should be pointed out that specific objec- 

 tives may require matching oculars. For 

 example, to compensate for the chromatic 

 undercorrection of apochromatic objectives, 

 certain eyepieces are overcorrected. Log- 

 ically these are called compensating oculars; 

 they should always be used with apochro- 

 mats. Other oculars which are intermediately 

 corrected are recommended by several man- 

 ufacturers to be used with their objectives. 

 Examples are Hj^perplane and Periplan 

 eyepieces. Oculars may be provided with net 

 reticules, scales, etc., to facilitate making 



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