22 Modern Microscopy 



vided with a rack and pinion for the coarse adjustment 

 of the object-glass, are made with the body to slide in a 

 fixed tube. This is a very rough-and-ready arrangement, 

 and accuracy of centring cannot be maintained as with a 

 rack and pinion. These instruments are, however, largely 

 used in hospitals and medical schools, and possessing one 

 element of advantage namely, cheapness other considera- 

 tions are often made subsidiary. Instruments of this kind 

 are generally cast aside or disposed of in favour of an in- 

 strument having rack and pinion after a very short time, 

 and anyone purchasing a microscope with a view of adding 

 apparatus to it would be well advised in having one with a 

 little less apparatus, but with a rack and pinion instead of 

 a sliding body. 



FINE ADJUSTMENTS. 



Well-defined attempts have been made by nearly all 

 makers to improve this most important of all movements. 

 The demand for accuracy in this particular has been greatly 

 increased by the growing use of objectives of large aperture 

 which cannot be profitably employed excepting with a fine 

 adjustment of the utmost precision. Its essentials are that 

 it impart a very slow motion and be absolutely free from 

 lateral movement. The fine adjustment that for many 

 years has proved thoroughly satisfactory in the writer's 

 hands is that made by Messrs. Watson and Sons and 

 applied to all their instruments. It is shown in position 

 on the instrument (Fig. 1) and the working details will be 

 gleaned from Fig. 5. 



The body is raised or lowered in a dovetailed fitting by 

 means of a lever contained within the limb of the instru- 

 ment, and a pin passing through it transversely acts as a 

 fulcrum. By turning a milled head attached to a micro- 

 meter screw, force is applied to the lever at one end against 

 a pointed rod, attached to the body and entwined by a coil 

 spring, at the other extremity. As the body moves upwards, 

 the spring is compressed against a brass plate, and on the 



