I] THE USE OF THE MICROSCOPE 5 



A mounted specimen is given you which is but 

 faintly stained. Focus it under the high power of the 

 microscope, as in 3 (6). Take away the diaphragm, 

 and notice that although there is more light in the 

 field of the microscope, the outlines of cells and fibres 

 are less sharp and distinct (especially with a sub-stage 

 condenser). Try diaphragms with different sizes of 

 aperture, and note which gives the best definition. 



When it is desired to see chiefly or solely stained 

 structures in the specimen no diaphragm should be 

 used ; the stained parts are then conspicuous, whilst 

 the unstained parts are barely seen 1 . 



5. Sub-stage Condenser. If the microscope has 

 a sub-stage condenser, raise this so that its upper surface 

 is level with the stage when using a high power; when 

 using a low power, lower the condenser till it is the 

 same distance below the specimen as the object-glass 

 is above it. 



6. Nose-piece. Screw about two-thirds of the 

 screw of the nose-piece into the microscope tube. (Do 

 this on the tube withdrawn from the microscope, if it 

 has a rack and pinion movement.) Then place the 

 nose-piece so that the free end is opposite the rack of 

 the tube, and screw the nose-piece home by turning 

 the screw in the nose-piece itself. 



Put on the two objectives. Focus a specimen with 

 the low objective. Turn the high objective slowly 

 round, note whether it touches the cover-glass ; if it 

 does, raise the tube a little before turning the high 



1 A teased osmic acid specimen of small medullated amongst non- 

 medullated nerves may be given to show this. 



