26 INTRODUCTION. 



on the cover-glass. Should the blurring be stationary in 

 both the above tests, it is due to soiling of the objective. 



To cleanse the lenses of the ocular, blow on both surfaces 

 of each lens and wipe dry with a fine silk handkerchief, 

 old soft linen rag, or, better, rice-paper. To cleanse the 

 objective, wipe, put the lens into the instrument and test 

 it as described in the preceding paragraph, and if this is 

 not sufficient, pass a little water over the surface and wipe 

 dry. If the soiling is due to balsam or other resinous sub- 

 stance, clean gently with benzole or xylol. The objective 

 system of lenses is held together by a shellac, soluble in alco- 

 hol, and all instrument-makers caution against the use of 

 alcohol on any part of the microscope. The back surface of 

 the objective need never get dirty ; but when it does, insert- 

 ing a soft rag into the objective and gently turning it around 

 is sufficient to cleanse it. Never screw apart the different lenses 

 of the objective, as it takes an expert optician *o 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 ^ m ch 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. 



A dark field illuminator, or condenser for dark ground illum- 

 ination, consists of a condenser in which the central portion 

 is darkened, and the rays of light, being admitted from the 

 circumference, are reflected to the centre by means of a mir- 

 rored edge. This arrangement produces a lateral illumina- 

 tion of the object against a black or grayish background. To 

 use this special condenser the ordinary Abbe" condenser must 

 be removed and the dark field illuminator substituted. A 



