THE MICROSCOPE. 137 



left, they should be removed by gentle breathing 1 on the 

 spot, and then slightly rubbing with clean, washed linen, 

 or with a piece of chamois skin. Alcohol should never be 

 applied to the lacquered surface, since it will remove the 

 lacquer. 



3. The milled parts on the objective, eye-piece, draw-- 

 tube, etc., should be handled and not the neighboring 

 smooth parts. 



4. The stage of the microscope should be kept clean 

 and dry. The glass slide, or plate, should never be placed 

 on the stage, unless the under surface is known to be dry. 



5. The mirror should be kept clean and free from dust. 

 The same is true of the Abbe condenser. 



6. The front lens of the objective must be kept per- 

 fectly clean. The No. 3 and No. 7 objectives are dry lenses, 

 and should never be allowed to touch oil, water, gelatin and 

 the like. If oil has touched the lens, it should be carefully 

 removed by means of a cloth moistened with a drop of 

 alcohol. Gelatin and agar can best be removed by touch- 

 ing with a cloth dipped in clean, warm water. The rV inch 

 objective is an oil immersion lens, and is never used dry. A 

 drop of cedar oil is placed between the front lens of this 

 objective and the clean, dry, upper surface of the cover- 

 glass. At the close of the day's work, the cedar oil must 

 be removed from the front lens by gently touching several 

 times with a well washed piece of old linen, or with a soft 

 tissue-paper. 



7 N The eye-pieces should be kept clean, free from 

 moisture and dust. 



8. At the close of the day's work, the microscope 

 should be carefully inspected, cleaned wherever it is nec- 

 essary, and placed in its locker. 



Measurement of an object. In this connection, it will be 

 well to describe the method pursued in measuring the size 

 of an object. It is necessary to have a stage micrometer 



