The Selection of a Microscope 45 



microscope by one of the well-known makers should be 

 selected, and the following would be the order of preference 

 for the various mechanical fittings : 



1. Coarse adjustment by rack-work, 



2. Fine adjustment. 



3. Compound sub-stage with screws to centre. 



4. Mechanical movements for the stage. 



5. Mechanical draw-tube. 



6. Fine adjustment to sub-stage. 



7. Concentric rotation to the stage. 



8. Divided scales, as may be found necessary. 



9. Other mechanical fittings, such as centring screws and 

 rack-work to the rotation of the stage, rack-work rotation 

 to sub-stage, etc. 



In amplification of the above we would remark that 

 where questions of economy prevail the sub-stage may be 

 replaced with an under-fitting having centring screws and 

 the mechanical stage with a sliding bar. 



Many microscopes are made in plain form as a founda- 

 tion on which as a super-structure many of the mechanical 

 fittings can be subsequently mounted ; consideration might 

 with propriety be given by a beginner to such instruments. 



TESTING A MICROSCOPE. 



The following are some points to be specially examined 

 when purchasing a microscope : The motions should be 

 perfectly smooth, with no lumpy feeling, and there should 

 be no backlash. This latter can be detected best by 

 holding gently the part that is actuated by the pinion, 

 and then attempting to rotate the pinion. If the pinion 

 rotates at all, or a movement of it can be detected with- 

 out a corresponding motion on the movable part, there is 

 backlash. Then, there should be no shake in any of the 

 fittings. In a badly-constructed microscope, even when the 

 fittings are in their most advantageous position, by holding 

 them and shaking them slightly, a movement in the slides 



