The Use of the Microscope 33 



and the whole unit is centered by the two screws shown in use in Fig. 38. 

 This arrangement is less flexible, and also a great deal less trouble, than 

 the mechanism preferred by Bausch and Lomb. The supplementary con- 

 denser lens, shown being inserted in Fig. 39, is mounted in a dovetail 

 slide. This is a great deal more trouble to use than the swing-out lens on 

 the Bausch and Lomb substage. 



Both substage mounts are provided with rack-and-pinion focusing, and 

 the particular American Optical model shown has in addition a fine ad- 

 justment. In Fig. 39 the coarse adjustment is the nearest milled head on 

 the right of the figure. The fine adjustment is the small milled head in 

 the center left of the picture. 



Mirror. The only requirement of the mirror on a research microscope 

 is that it should be flat and easy to remove or turn aside. 



Objectives. The choice between apochromatic and achromatic objec- 

 tives, the difference between which was explained in Chapter 1, depends 

 on the use to which the microscope is to be put. Achromatic objectives 

 have many advantages. They are cheap, accurately parfocal, of long work- 

 ing distance, and of sufficiently high N.A. for most purposes. Apochromatic 

 objectives have only the advantage of high resolution, more usually re- 

 quired for photographic than for visual work. It must be remembered 

 that all four lenses on the turret must be in the same class, since special 

 (compensating) eyepieces are required for apochromatic objectives. The 

 image given by an apochromatic objective with a regular eyepiece is far 

 inferior to that given by an achromatic objective with the same eyepiece. 



No matter what class of objective is chosen, the instrument should be 

 provided with XlO, X20, X40, and X90 (oil-immersion) objectives. A 

 X3.5 objective is even less practical on these instruments than on a medi- 

 cal microscope. In the apochromatic range there is often a choice be- 

 tween a X90 N.A. 1.3 oil immersion and a X95 (or XlOO) N.A. 1.4 oil 

 immersion. Only in the rarest circumstances does the 7 per cent increase 

 in the theoretical resolution of the N.A. 1.4 justify the increased cost. 

 Those who must have such a lens, because almost all of their work lies 

 in the examination of objects at the extreme limit of the resolution of the 

 optical microscope, would do well to substitute a X80 (3-mm) oil im- 

 mersion for the X 10 lens on the turret. These lenses have a field of view 

 almost twice that of the N.A. 1.4 and are used to search the field for, 

 and center, the object for subsequent examination under the high-resolu- 

 tion lens. 



Oculars. These must be purchased as matched pairs. Although XlO 

 and Xl5 are all that most people use, under ideal conditions of illu- 

 minating and centering the image made by an N.A. 1.4 apochromatic im- 

 mersion lens will stand examination under a X20 ocular. There is no 

 point in using a X20 ocular with lower-power objectives. A X20 ob- 



