THE MICROSCOPE AND ITS PARTS. 37 



the cell without disturbing it, and so taking the risk of 

 losing the invisible thing. He may also wish to watch 

 its growth and development. A reservoir for a water- 

 supply is necessary; an "individual" butter-dish makes 

 a good one. Place the slide across the dish, apply a 

 doubled thread of sewing-cotton along one side of the 

 square cover, so that each end shall hang down into 

 the dish, and fill the latter with water, which will then 

 pass up along the thread, and keep the cell full for as 

 long as may be desired. The only objection to this 

 little affair is that, after a few days' use, the salts 

 in the water will crystallize on the cover, and so pre- 

 vent the absorption of oxygen from the air. But no 

 growing-cell is free from some objectionable features; 

 none can quite imitate natural conditions, and the 

 animal or plant dies before long, either falling to 

 pieces, or becoming buried beneath a mass of fungi. 

 This form 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 absolute con- 

 tact 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 crystal must not he forgotten. But 

 often, in lowering it over the wet specimen, small 

 bubbles of air will be caught and not noticed until 

 magnified, when, if seen for the first time, they appear 

 wonderful, if not startling. Some strange statements 

 have been made and discoveries announced whose only 

 foundation has been minute air-bubbles which the ob- 

 server did not recognize. A man once described a 



