MICROSCOPIC EXAMINATION. 475 



ment until the lens just touches the oil ; then it is accurately adjusted 

 upon the preparation with the micrometer screw. 



(a) Unstained preparations. Narrow diaphragm! They are exam- 

 ined in two ways : 



1. A drop of pure culture in a fluid medium or a little drop of water 

 with a trace of pure culture mixed in it is placed between the slide 

 and cover-glass; or, better, 



2. In a hanging drop. A drop of a pure culture in a fluid medium 

 or a drop of bouillon in which is mixed a minute quantity of a pure 

 culture is placed upon a cover-glass ; the cover-glass is then turned over 

 and placed upon a hollow ground slide so that the drop is suspended 

 within the hollo\v. The cover-glass is now fixed to the slide by apply- 

 ing a very little water to each corner of the cover, or, if the observa- 

 tion is to be more prolonged, by means of vaselin. 



(b) Stained preparations. 'Open diaphragm! Abbe's illuminating 

 apparatus. In the examination of sections with a double stain, the 

 wide diaphragm is required for the bacteria, the narrow opening for 

 the tissues. 



(C) Cleaning the Preparations and the Microscope. The 

 immersion oil is always gently brushed off, and now and then quickly 

 cleaned with xylol and chamois skin; the setting of the lens is loosened 

 by prolonged action of the xylol. Also immersion oil dried upon the 

 cover-glasses of old preparations is readily removed by xylol. 



2. The most Important Solutions for Use in Making 

 Preparations. 



(A) Staining Solutions. 



1. Aqueous Alcoholic Solutions of Fuchsin and Methylene- 

 blue. A concentrated " stock -solution " is prepared by pouring abso- 

 lute alcohol upon the pulverized dyes (fuchsin, methylene-blue) in 

 bottles, and after shaking and allowing them to stand a few hours, 

 they are filtered. Of this saturated solution 1 part is mixed with 4 

 parts of distilled water, and before using is filtered. In order to 

 obtain good preparations it is better to stain a longer time with weaker 

 solutions than for a short time with strong solutions. 



2. Carbol-fuchsin (Ziehl's Solution). 



Fuchsin 1.0 gm. 



Acid, carbolic, liq 5.0 u 



Alcohol 10.0 " 



Aq. dest 90.0 " 



3. Anilin Fuchsin. Four parts of anilin oil (anilin. pur.) are well 

 shaken several minutes with 100 parts of distilled water, and then fil- 

 tered until all the water has passed through clear (then the funnel is 

 removed, since otherwise oil will also pass through). In this anilin 

 water 4.0 gm. fuchsin are dissolved and it is again filtered. 



4. Anilin Gentian-violet (Ehrlich's Solution). To 100 c.c. of 

 anilin water, 11 c.c. of a concentrated alcoholic solution of gentian- 

 violet (stock solution) is added. This solution does not keep long. 



