40 MICROSCOPY FOR BEGIXXERS. 



objectionable features ; none can quite imitate the natu- 

 ral conditions, and the animal or plant dies before long, 

 either falling to pieces or becoming buried beneath a 

 mass of fungi. This one will supply an abundance of 

 water, if the water in the dish is always kept in contact 

 with the lower surface of the slide. This, and the ab- 

 solute contact of the thread with the edge of the cover, 

 are the only things whose absence will result in defeat. 

 As the reader already understands, the object must 

 never be examined in water without being covered by 

 either a thin glass circle or square ; the importance of 

 this little piece of glass must not be forgotten. But 

 very often, in lowering it over the wet specimen, small 

 bubbles of air will be caught and not noticed until mag- 

 nified, when, if seen for the first time, they appear won- 

 derful, if not startling. Some strange statements have 

 been made, and discoveries announced whose only foun- 

 dation has been minute air-bubbles that the observer 

 did not recognize. A man once described a marvellous 

 something that he had found in a cancer, 

 which turned out to be a magnified air- 

 bubble. These little air-drops always 

 play an amusing part at the beginning 

 of the microscopist's career. In Fig. 4 

 are shown several of different sizes. Let 

 Fig. 4. Air-bubbies. the student examine a drop of saliva or 

 of soapsuds, and he will in future be able 

 to recognize the troublesome things. Pictures or words 

 cannot convey so true an idea of their appearance as a 



