94 



THE MICROSCOPE 



Joint. 



Limb. 



and must work with a smooth, even motion that allows of the 

 most delicate setting for focus. The coarse adjustment should 

 be capable of focussing with a 1/6-inch object glass, although 

 after the focus has been found the slow motion will generally be 

 used. It is an advantage to have a series of divisions on the slow 

 motion by which the thickness of a cover glass or a section can 

 be ascertained (see page 53). The value of the divisions is given 

 under the description of different microscopes. The two adjust- 

 ments should work in fittings which are made with the utmost 

 precision. These fittings should be solid metal slides without 

 any adjusting screws. The wear in the fittings of a microscope 

 is infinitesimal compared with those of running machinery, and 

 slides well fitted in the first instance will wear for a lifetime 

 without adjustment if properly used. All kinds of adjustable 

 fittings have been tried, but have been abandoned. The adjusting 

 screws work loose, the slides do n6t have to be so well fitted 

 originally, and nothing is so good as a solid slide well fitted in 

 the first instance. 



The milled heads of both the adjustments should move in the 

 same direction, so that the upper portion of the milled heads is 

 going away from the observer when the body tube is going down 

 or approaching the object. Mistakes made by turning the milled 

 heads in the wrong direction may result in breaking the slide or 

 damaging the object glass. If all the milled heads in a microscope 

 move in one direction, such mistakes need not be made. 



A microscope should have a 

 joint for inclination. The instru- 

 ment may have to be used occa- 

 sionally in a vertical position, but 

 it is so much more convenient in 

 any other position that an in- 

 clining joint should not be omitted 

 from the stand. The writer does 

 all his most difficult tests and ex- 

 aminations with a microscope on 

 an optical bench in an almost hori- 

 zontal position, the axis pointing 

 down only about 15°. The eye- 

 piece is at the eye-height of the 

 observer when in a sitting position. 

 Prolonged observation of several 

 hours ceases to be tiring with the 

 microscope thus arranged. An 

 ordinary microscope, however, in- 

 cUned to about 45° is very com- 

 fortable for prolonged work. 

 A microscope should have a limb that can be readily grasped 

 by the hand for lifting. It must never be lifted by its body or 



Fig. 91. 



