METHODS OF OBSERVING BACTERIA. 93 



For ordinary staining an aqueous solution made in a 

 simple manner is employed. A small bottle is nearly 

 rilled with distilled water, and the stock-solution is added, 

 drop by drop, until the color becomes just sufficiently in- 

 tense to prevent the ready recognition of objects through 

 it. Such a watery solution possesses the power of readily 

 penetrating the dried protoplasm of the bacterium, taking 

 the stain with it. Alcohol does not have this power. 



As in the process of staining the cover is apt to slip 

 from the fingers and spill the stain, it is well to be pro- 

 vided with cover-glass forceps (Fig. 8), which hold the 



FIG. 8. Stewart's cover-glass forceps. 



glass in a firm grip and allow of all manipulations with- 

 out danger to the fingers or clothes. The ordinary in- 

 struments are entirely unfitted for the purpose, as capil- 

 lary attraction draws the stain between the blades and 

 makes certain the soiling of the fingers. Sufficient stain 

 is allowed to run from a pipette upon the smeared side 

 of the cover-glass to flood it, but not overflow, and is 

 allowed to remain for a moment or two, after which it 

 is thoroughly washed off with water. If the specimen 

 is one prepared for temporary use, it can be examined at 

 once, mounted in a drop of water, but under these con- 

 ditions will not appear as advantageously as if dried and 

 then mounted in Canada balsam. 



Sometimes the material to be examined is too solid to 

 spread upon the glass conveniently. Under such circum- 

 stances a drop of distilled water can be added and a minute 

 portion of the material be mixed in it upon the glass. 



The entire process is, in brief : 



i. Spread the material upon the cover ; 2. Dry do not 

 heat ; 3. Pass three times through the flame ; 4. Stain 



