8 HOW TO WORK 



altering the distance between the front and second pair of glasses. 

 An engraved line shows the point to which the lens should be set 

 for uncovered objed Its adjustment for covered objects is to be 

 effected in the following manner : arrange the objective as if for an 

 uncovered object; then any object covered with thin glass is brought 

 into focus by moving the body of the microscope ; next the milled 

 adjustment ring adapted to the object-glass is turned round until any 

 particles of dust upon the upper surface of the thin glass covering the 

 object are brought into focus. The lens is thus ''corrected'''' for the 

 thickness of the cover, and it only remains to re-focus the object. 



The mechanical arrangement usually employed in this country 

 for " correcting " is unsatisfactory. The screw usually works too 

 hard, and the thread is too coarse. Mr. Wenham has introduced a 

 great improvement, which entirely overcomes these objections, and 

 enables the observer to " correct " from time to time while he is 

 examining the object. The middle and posterior lenses are made to 

 alter their position instead of the front lens. This is a very 

 valuable improvement. 



10. The Mirror, pi. V, fig. 19, should slide upon an upright 

 beneath the stage, so that it may be arranged near to, or at 

 a distance from, the object, and it should be capable of being 

 inclined at any angle, so that rays of light may be reflected from it 

 and made to pass directly through the object in straight lines, or 

 thrown upon it in a very oblique direction. The mirror should be of 

 full size, one surface quite plane and the other concave, so that a 

 strong light may be condensed upon the object when required. The 

 achromatic condenser and other pieces of apparatus of advantage for 

 examining objects by transmitted and reflected light are described in 

 Part II. 



Mechanical Portion of the Microscope. 



In directing attention to the mechanical arrangements of the 

 microscope, I must say a few words upon the adjustments for alter- 

 ing the focus, the body of the instrument, and the stage. 



11. Adjustments for altering the Focus. The ordinary move- 

 ment is obtained by the rack and pinion. In some microscopes 

 the body is moved by the fingers alone, and is arranged to slide in a 

 tube (which may be lined with cloth) like a telescope. In the instru- 

 ments of Mr. Ladd the requisite motion is obtained by the ordinary 

 milled head, while delicate focussing is carried out by a lever, but the 

 movement is effected by a chain instead of rack and pinion. Besides 

 coarse adjustment, however, every microscope should be provided 



