A SIMPLE DESCRIPTION OF THE MICROSCOPE 19 



motion for focussing with object glasses of lower power than 

 1/6-inch (4 mm.). 



The fine focussing adjustment does exactly the same as the Pine 

 coarse adjustment, but the movement is far more delicate : aa°i'Smlnt. 

 it is actuated by a micrometer screw and a lever moving the whole 

 body along a second slide. A complete turn of the screw moves 

 the body about a quarter of a millimetre. Turning the fine 

 adjustment miUed heads moves the body in the same direction 

 as those of the coarse adjustment. In the " Standard " micro- 

 scope the left-hand milled head is twice as delicate a motion as 

 that on the right-hand side. The fine adjustment is required 

 for the focussing of high powers and for ezamining the difierent 

 layers of an object. 



In moving the body of the microscope up and down to obtain The best 

 the correct focus, care is required to prevent the front of the Sou^i'^g' 

 object glass being forced into contact with the object by racking 

 it too far down. It is easy to break a valuable specimen by this 

 means; and although for its protection the metal mount of 

 the object glass projects slightly in front of the front lens, it is 

 delicate in construction, and can be damaged by being brought 

 into contact with the specimen. 



Experienced microscopists can focus a lens downwards and 

 stop at the position where the object is sharply seen, but it is 

 unsafe. The correct method is to set the body of the microscope 

 so that the front of the object glass almost but not quite touches 

 the object, and then to rack backwards, turning the milled heads 

 so that the upper portion turns towards the observer, and raise 

 the body imtil the correct focus is found. With high-power 

 object glasses, especially oil-immersion lenses, this method is 

 not so easy because the distance of the correct focus may be 

 below the point at which the body has been set in the first instance. 

 If, however, the slow motion is used to make the final adjustment, 

 damage is not likely to occur, as it lowers the body very gradually, 

 and the latter is only pressed down upon the object by a spring. 

 When using an oil-immersion lens ajdrop of cedar-wood oil should 

 be placed on the object glass, and the body of the microscope 

 racked down imtil the drop of oil touches the cover ; the final 

 focussing can then be done with the^ne adjustment. 



Some objects are so transparent that it is quite easy to pass 

 by the focus and miss the correct position. In these cases dust on 

 the cover glass maybe focussed first,and the fine adjustment lowered 

 by an amount representing the thickness of the cover glass. If 

 the slide be moved backwards and forwards on the stage during 

 the process of focussing, the movement will be seen directly the 

 correct position is nearly reached. 



It may appear absurd to mention that if a slide happens to 

 have been placed on the stage upside down a high- power object 

 glass will not focus through the thick glass slip, but the writer 



