68 BACTERIOLOGY 



found to have so much moisture as to render the addition 

 of water superfluous. If it is desired to examine the juice 

 of organs, a piece of the organ is seized in a forceps and 

 the cover-glass smeared with it, dried in the air, and passed 

 three times through a flame to fix the micro-organisms to 

 its surface, after which the staining is done by depositing a 

 few drops of dye on the infected surface of the cover- 

 glass, or by pouring some into a watch-glass and floating 

 the cover-glass upon the solution with the prepared side 

 downwards. After from one to five minutes it is freed from 

 superfluous stain by washing in water, is dried in the air 

 — a process facilitated by soaking up the drops with blotting- 

 paper — and is mounted in fairly fluid Canada balsam ; or, 

 if it is not wished to preserve the preparation, it may be 

 examined in water, or in a very dilute solution of potassium 

 acetate. Such objects are examined with ordinary or 

 homogeneous immersion objectives by the aid of Abbe's 

 illuminating apparatus without a diaphragm. Coloured 

 preparations admit of being seen with distinctness, and 

 their outlines can be accurately determined, such figures 

 being spoken of as ' coloured images,' to distinguish them 

 from the unstained ' structural images,' which should 

 only be examined with a diaphragm of narrow aperture. 

 When Abbe's apparatus is used without a diaphragm, all 

 the rays which enter the lower lens, and which form 

 a very obtuse-angled pencil, are enabled to reach the 

 object. 



Bacteria are difficult to observe in fluids and tissues, 

 being only visible through the shadows caused by the 

 diilerences in refractive power of the several structures. 

 Hence but little light must be allowed to reach the prepara- 

 tion, and consequently as small a diaphragm as possible 

 used, and the result is an impairment of distinctness. If, 

 however, the bacteria be stained, it becomes possible to 



