22 Modern Microscopy. 



are a necessity. A scale, reading to parts of a millimetre or 

 inch on the plates of the mechanical stage, will be found 

 useful, as important parts of an object can be noted and 

 refound. For instance, supposing a specimen were being 

 examined, and any important feature observed to which 

 future reference would be convenient, the reading of the 

 divisions on the stage would be marked on the slide say, 

 horizontal, 24 ; vertical, 20. On future occasions it would 

 only be necesteary to set the stage readings at the same 

 points and place the object in the same position on the 

 stage (for which purpose nearly all mechanical stages have 

 a stop-pin for the slide to be set against), and the special 

 feature will be in the field of view. If a mechanical stage 

 be selected, it should be a good one, as if badly made it is 

 worse than a plain stage ; also the frictional parts should 

 be fitted with adjusting screws to compensate for wear and 

 tear. 



For a stage for a microscope we would therefore place 

 our choice as follows : 



For a first-class microscope : Mechanical movements ; 

 concentric rotation ; screws to make the rotation quite true 

 with any objective; sliding-bar to top plate and stop-pin for 

 object to go against ; divisions to plates of stage reading 

 to parts of millimetre or inch ; rackwork rotation to stage ; 

 and divisions to outer circle (optional). 



For a second-class microscope : Mechanical movements ; 

 sliding-bar to top plate. 



For a third-class microscope : Plain stage, with springs 

 to hold object in position ; if provided with sliding-bar or 

 plate as object-carrier, so much the better. 



THE SUB-STAGE OR FITTING UNDER THE 

 STAGE TO CARRY CONDENSER, ETC. 



The sub-stage consists of a ring of what is termed the 

 ' society size,' 1J inches full internal diameter, which 

 carries illuminating apparatus for condensing the light on 



