32 TEXT-BOOK OF BACTERIOLOGY. 



visible, their outlines are clear and sharp ; in short, the whole pic- 

 ture is now adapted to the purpose of bacterial investigation. 



The reason is that under ordinary conditions the lines and 

 shadows which compose the structure picture are liable to darken 

 and cover up stained objects of small dimensions, which therefore 

 do not emerge from their obscurity until the structural part of 

 the picture disappears. Herein lies the value of Abbe's illuminating 

 apparatus : it is capable of giving decided prominence to the colored 

 portions of a stained preparation, particularly to the nuclei and the 

 bacteria. 



This may be elucidated still further by another example. The 

 eiTect of Abbe's apparatus may be considerably diminished (and 

 even quite destroyed) by diminishing the base of the cone of rays 

 with the aid of diaphragms, or "stops," and thereby decreasing its 

 angle of aperture. The smaller the opening of the diaphragm, the 

 more it shuts off the action of the condenser, and with a very small 

 opening one works, so to say, without the Abbe. 



Examine a preparation of micrococci treated with fuchsin first 

 without any diaphragm — i.e., with the full working of the Abbe and 

 the isolation of the color picture. A collection of micrococci will 

 appear as small, uniform, strongly -colored grains. The close obser- 

 vation of them will not be difficult. 



If a small stop be employed — that is, the Abbe shut off — the struc- 

 ture of the tissue immediately becomes visible, and the bacteria 

 which before were so distinct are now indistinct and thrown into 

 the shade, so that with all possible efforts they cannot be found 

 again. Nevertheless the place where they ought to be visible is 

 known. How much more unfavorable, then, would be the circum- 

 stances if the search had to be made without even this aid. 



All colored bacteria preparations in which the effect of the 

 colored picture alone is advantageous must be examined with the 

 undiminished action of the Abbe — i.e., without any stops — but all un- 

 stained objects in which only the structure picture is desired must 

 be examined with the smallest possible stop or opening. 



By the words "smallest possible" stop is understood a stop 

 which leaves a sufficient illumination of the field ; as a rule, the 

 ' higher the power employed the larger must the stop be. 



Special note should be taken of this. Beginners often err from 

 inattention to the rules just given, and they are seen, faithful to 

 their histological custom, either examining stained preparations 

 with the diaphragm, or uncolored objects, hanging drops, etc., 

 under the full effect of the Abbe, without any stop. Under both 

 circumstances, little can be seen of what it is desired to recognize. 



