MICROSCOPICAL TECHNIQUE 41 



In using the immersion lens a drop of cedar oil is placed 

 on the cover glass, care being taken to avoid air bubbles. 

 The tube of the microscope is lowered until the lens touches 

 the drop and is nearly in contact with the cover glass. This 

 must be done with care, for if the tube is lowered by the 

 coarse adjustment until the objective touches the cover glass, 

 a broken preparation or a damaged lens is likely to result. 

 In some of the modern types of microscopes the fine adjust- 

 ment is so arranged that its action is stopped when the 

 objective touches the preparation. 



At the end of the day's work the oil must be removed 

 from the lens by wiping with Japanese lens paper, which 

 should be kept in an envelope or glass jar to protect it from 

 dust. If the lens becomes sticky by the drying of the oil, 

 the latter must be removed by saturating a piece of lens 

 paper with xylene and wiping the lens carefully. An excess 

 of xylene must be avoided, as it is a solvent of the lens 

 mounting. After the treatment the lens should be dried 

 with clean paper. 



Slides and cover glasses. The American slide is 25 x 

 75 mm. of clear white glass. The cover glasses used for bac- 

 teriological work must be thin on account of the short work- 

 ing distance of the higher-power objectives. No. 2 cover 

 glasses are usually employed, having a thickness of 0.17 

 0.25 mm. The slides may be cleaned by washing in water or 

 alcohol and drying with a towel. The cover glasses must be 

 wholly free from fat, in order that an even distribution of the 

 liquid to be examined may be obtained. New cover glasses are 

 cleaned by washing in water, and then immersing in alcohol 

 to which 3 per cent of hydrochloric acid has been added. The 

 cover glasses are dried by rubbing between driers made of 

 wooden blocks 200 X 100 X 25 mm., covered with several 





