184 THE MICROSCOPE. 



for cleaning cover-glasses, slides, etc. If a fresh piece is used 

 each time for cleaning an objective, there will be little danger of 

 marring the polish of the front lens. 



Keep the bottles containing reagents, staining fluids, etc., 

 stopped when not in use, so as to exclude dust and prevent 

 evaporation ; and take especial care in using them not to intro- 

 duce impurities into them. 



8. In cutting sections; the razor or section-knife should al- 

 ways be very sharp, and for most purposes sections should be 

 cut quite thin. Steadiness in cutting is secured by resting the 

 wrists against the body. The knife should be given an oblique 

 or sliding motion when drawn through the object. Quite hard 

 tissues may be successfully cut if the sections are cut quite thin, 

 but if the edge of the knife is allowed to run too deep, it is lia- 

 ble to be notched. Portions of thin structures like leaves, pet- 

 als, etc., may readily be sectioned by placing them between 

 pieces of Elder or Sunflower pith. Always clean the knife after 

 cutting with it, and do not allow fragments of tissue to dry upon 

 it. Sections of fresh tissue?, or of those that have been pre- 

 served in any of the preservative fluids, should immediately be 

 placed in liquid after cutting, to preventair from getting into the 

 cells. 



9. In all your work with the microscope, proceed under- 

 standing^. Endeavor to know the reason for every test you 

 are called upon to apply, and be careful in interpreting the re- 

 sults of each test. Do not rest satisfied until you thoroughly 

 understand every tissue and structure given you for study. 



10. It is excellent practice for the student to keep an accu- 

 rate record in writing, and by means of drawings, of all the facts 

 observed and the work done by him in the microscopical labor- 

 atory. 



Flies and Contagion. — A fly was infected with cholera and 

 allowed to feed upon some beef. The meat was in due time 

 found swarming with the bacteria. If Cholera ever gets a start 

 in Washington it will run riot, for there are, every summer, in 

 this city more flies to each acre than can be found in all Paris. 

 Only those who screen the flies out of their houses may expect 

 to escape and even then contagion may come in on meat and 

 pther eatables. Sufficient heat, however, will sterilize the food, 



