4 PRACTICAL HISTOLOGY [l 



If the specimen is transparent this method is much 

 safer than (a). 



Great care should be taken to avoid lowering the 

 objective upon the mounted specimen, since this may 

 force the front lens out of position. If the front lens 

 is thus decentred, the objective should be sent back to 

 the maker to be re-set. The high power should not bo 

 used for thick objects. 



When the section is very transparent, the edge of 

 the cover-slip supposing this is not surrounded by a 

 projecting rim of balsam or glycerine may first be 

 focussed to find the level of the cover-slip, and. then 

 the slide moved about till a faint shadow of the section 

 is seen. 



Before removing the slide from the stage, raise the 

 microscope-tube, in order to avoid the chance of the 

 front lens being brought into contact with mounting or 

 cementing material at the edge of the cover-slip. In no 

 case must a second specimen be placed under the high 

 power without raising the microscope-tube, since the 

 glass slides are not of constant thickness. 



The microscope should never be left with the ob- 

 jective on, and the ocular out, or dust will settle on the 

 upper lens of the objective. 



After using the microscope put it back in the box, 

 or cover with a bell-jar. 



The student should accustom himself to keeping 

 both eyes open when using the microscope. 



4. Use of diaphragm. A diaphragm is used to 

 cut out the peripheral rays of light which are not 

 brought to the same focus as the central ones. 



