26 INTRODUCTION. 



scribed in the preceding paragraph, and if this is not suffi- 

 cient, pass a little water or absolute alcohol over the surface 

 and wipe dry. If the soiling is due to balsam or other resin- 

 ous substance, clean gently with benzole or xylol. The back 

 surface of the objective need never get dirty ; but when it 

 does, inserting a soft rag into the objective and gently turn- 

 ing it around is sufficient to* cleanse it. Never screw apart 

 the different lemes of the objective, as it takes an expert optician 

 to put them into proper position. Always see that the cover- 

 glass is clean and dry on its upper surface. Never bring 

 the front lens of the objective into direct contact with the 

 object or 'cover-glass. 



For bacteriological work, it is indispensable to have a 

 microscope supplied with an Abbe condenser and an oil-im- 

 mersion objective of -fa inch focus. 



Different objectives according to their construction require 

 different tube-lengths of the microscope to magnify at their 

 fullest power and give their best definition. Manufacturers 

 generally supply full information as to the proper tube-length 

 for each instrument. 



QUESTIONS. 



What is refraction ? 



Give the two laws of refraction. 



What is the type of the microscope lenses ? 



What is the focus of a lens? 



What is meant by spherical aberration ? How is this corrected ? 



What are doublets and triplets? 



What is meant by refrangibility ? How is this corrected in microscope 

 lenses? 



How many kinds of microscope are there? 



What is a simple microscope ? 



How is the magnifying power of lenses expressed? 



What is meant by a compound microscope ? 



Give the different parts of a compound microscope ? 



What is meant by direct light? 



What purpose does a diaphragm serve? 



What is a condenser? 



What is meant by an iris diaphragm ? 



What is a nose-piece ? 



How are the lenses of an ordinary microscope called ? 



What is an objective ? 



How many lenses or combinations of lenses does an ordinary objective 

 contain ? 



How is the magnifying power of objectives designated? 



