SLIDES AND COVER-GLASSES 131 



Slides similarly should be carefully washed in alcohol and dried. 



B. Slides and cover-glasses can be used over and over again. They must 

 however be carefully cleaned to remove all traces of material on them ; 

 unless this be properly done mistakes are likely to occur when they are used 

 a second time. The thorough cleaning of soiled slides is therefore of great 

 importance and can be done as follows : 



1. Drop all used slides and cover-glasses when they are finished with into 

 a dish containing spirit. 



2. When a number have collected put them into a porcelain dish, cover 

 them with a 4 per cent, solution of sodium carbonate and boil for half an 

 hour. 



3. Pour off the soda solution, wash in a large volume of water, then drop 

 them into the following solution : 



Water, - - 1000 grams. 



Potassium bichromate, - - 50 



Sulphuric acid, - - 100 



and boil again for half an hour. 



4. Pour off the bichromate solution, wash again in a large volume of tap 

 water, then in distilled water, wipe them dry and drop them one by one into 

 covered pots filled with 95 per cent, alcohol, out of which they can be taken 

 as required. 



This method will ensure the glasses being clean. 



2. Method of using cover-glasses and slides. 



Cover-glasses should be picked up by one of their angles with a pair of 

 Cornet's (fig. 118) or Debrand's (fig. 119) forceps. 



FIG. 118. Cornet's forceps. 



Debrand's forceps, a very useful modification of Cornet's, are well balanced 

 and easily held in the hand : they give a firm hold and do not break the 



cover-glasses. 



FIG. 119. Debrand's forceps. 



SECTION II. THE EXAMINATION OF UNSTAINED PREPARATIONS. 



A little drop of a culture of a micro-organism may be mounted between a 

 slide and cover-glass and examined. But to keep the organisms alive while 

 they are being examined for the purpose of studying the method of multiplica- 

 tion, etc., special slides having a small concavity or cell ground in their centre, 

 are used. A drop of broth is placed in the cell and sown with the organism ; 

 in this way a living culture is available for the purposes of microscopical 

 examination. 



