THE MICKOSCOPE 7 



more than ever necessary for this movement to be as slow 

 as possible consistently with convenience in working, since 

 owing to the large aperture of the objectives now obtainable 

 small focal changes have a much greater effect on the image 

 than formerly. 



The microscope should be provided with a draw-tube, 

 preferably actuated by rackwork, although this is not abso- 

 lutely necessary ; the length of the draw-tube will be dependent 

 on the objectives that are selected, but it is preferable, if 

 possible, to have sufficient length of tube so that either ' short ' 

 or ' long ' tube objectives, as they are called, can be used at will. 



The stage should be large, and very firmly supported, so 

 that no light pressure on it such as occurs when an object is 

 being moved about on the stage will result in flexure and 

 consequent displacement of the object in relation to the optical 

 system. The stage should have a large hole in it ; or, better 

 still, an elongated slot with its opening on the side away from 

 the observer. The opening in any case should always be 

 sufficiently large to allow for any sub-stage illuminating 

 apparatus that may be adopted. 



A mechanical stage is of course very convenient, but by no 

 means essential. If, however, it is adopted, it should be of the 

 very best possible design and construction. A bad mechanical 

 stage is worse than useless, and will only lead to trouble. It 

 should work quite smoothly and exactly at right angles to the 

 optical axis. If the moderate additional expense is not a 

 consideration, then verniers for determining the exact position of 

 any particular spot in an object, so that a record of its position 

 may be kept, are often of great utility. Except for very high- 

 power work where long searches for particular appearances are 

 necessary, the writer prefers to use an instrument with a large 

 flat stage, and without any mechanical arrangement for shifting 

 the object ; certainly, unless a mechanical stage of the very best 

 make can be obtained, this is the better method. A centreing 

 stage that is, one in which an arrangement is made for a 

 very small movement of the stage to facilitate the exact 

 centreing of an object is exceedingly useful in setting up 

 objects for photographing. 



The points so far enumerated are the principal ones in the 

 part of the instrument that lies above the stage ; but the 



