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 lenses 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? 



