The Stage 13 



fixed centre of the base-plate, the mechanical movements 

 acting above it. It is then termed a concentric rotating 

 stage, the object remaining in the field during the whole 

 rotation of the stage. In mechanical stages of economical 

 construction the rotating plate is occasionally fitted above 

 the mechanical movements, and is carried by them, in which 

 case it does not rotate concentrically. The object can, not- 

 withstanding, be kept in the centre of the field by constantly 

 re-setting it with the mechanical screws during the rotation 

 of the plate. Some stages of the concentric form are 

 arranged to rotate by rackwork and pinion ; although this 

 is not really an essential, it is often convenient ; it also 

 prevents the stage from rotating accidentally, especially 

 in photography. When it is provided, it should have the 

 pinion-wheel so arranged that it may be disengaged from 

 the rack and replaced instantly. 



Centring screws to the concentric rotating stage, by 

 means of which the axis of the stage may be made true with 

 any objective, will be found a useful addition, especially 

 if petrological work is to be done. Divisions to the 

 •periphery of the stage for reading the angle through which 

 the stage is rotated are not advantageous for ordinary 

 purposes, but for chemical and petrological work they are 

 a necessity. 



Finders to Mechanical Stages. — Divided scales, read- 

 ing to parts of an inch or millimetre, fitted to the plates 

 of the mechanical stage, will be found of great utility. By 

 means of such an arrangement, importabt parts of an 

 object can be noted and subsequently refound. For instance, 

 supposing a specimen were being examined, and an im- 

 portant feature were observed to which future reference 

 would be desirable, it would only be needful to take the 

 reading of the divisions on the stage, and record them on 

 the slide — say, horizontal, 24; vertical, 20. On future 

 occasions, on setting the stage readings at the same points 

 and placing the object in the same position on the stage 

 (for which purpose nearly all mechanical stages have a 



