90 A MANUAL OF BACTERIOLOGY 



microscope for thicker cover-glasses. Zeiss makes 

 a good tester (Fig. 27) suitable for the exact 

 measurement of the thickness of cover-glasses. The 

 measurement is effected by a clip projecting from a 

 box ; the reading is given by an indicator moving 

 over a divided circle on the lid of the box. The 

 divisions show hundredths of a millimetre, and the 

 instrument is capable of measuring up to five milli- 

 metres. 



Before use the glass slides and cover-glasses should 

 be perfectly clean. 



Many methods for permanently mounting tissues 

 and cover-glass preparations have already been 

 described. For fresh tissues glycerine is often used, 

 while for hardened tissues the following mounting 

 media have each their special advantages : 



(a) Canada balsam dissolved in xylol. 



(&) Canada balsam dissolved in benzol. 



(e) Canada balsam dissolved in chloroform and 

 turpentine. 



(d) Dammar varnish. 



After the tissues have been stained, they pass 

 through the following processes : Washing off the 

 excess of stain, dehydrating, clearing or extracting 

 the infiltrated material used in the imbedding pro- 

 cess, etc. ; mounting, cementing, or sealing ; and 

 finally, labelling the slides. The following list gives 

 the various agents for the above-mentioned pro- 

 cesses : 



(Water. 



(1) Washing agents, . . . J Dilute spirit. 



I Absolute alcohol. 



(2) Dehydrating agent, . . Absolute alcohol. 



