TEXT-BOOK OF BACTERIOLOGY. 33 



By this we do not mean that in all cases and under all circum- 

 stances the stained preparations are to be only examined without 

 stop. On the contrary, such an examination is only advisable for 

 the beginning- of an investigation, when we wish to know whether 

 our object contains any micro-organisms at all, and if so, what 

 forms, what appearance, and what arrangement they present. 

 This ascertained, the next questions are : What relation do the bac- 

 teria bear to the tissues and how are they distributed in them ? 



These questions can only be answered with the aid of a suitable 

 stopping. The structure picture must be allowed to reappear so 

 far as is compatible with the sufficient preservation of the delicate 

 color picture and without hiding the bacteria. It must be the aim 

 of every careful observer to hit this happy medium between the 

 extremes. The selection of a diaphragm and the consequent illumi- 

 nation of the preparation is greatly facilitated by an instrument 

 first applied in England, but with which most microscopes are now 

 provided, called the iris diaphragm. 



The alteration which must be given to the pencil of rays pro- 

 ceeding from the angle of aperture of the condenser is here ob- 

 tained with certainty by simply moving an arm which regulates 

 the size of the opening, and the distance to be moved must be de- 

 termined by the appearance of the microscopic picture. 



II. MICROSCOPICAL EXAMINATION OF BACTERIA. 



In the present state of science and its auxiliary machinery 

 bacteria can be examined unstained or stained. 



The former is the simpler process, and though it only yields-, 

 satisfactory results to a certain limited extent, yet it is an ex- 

 tremely essential and altogether indispensable part of our investi- 

 gations. 



Never be committed to anything approaching an expression of 

 certainty with regard to a species of bacteria before examining it 

 in its uncolored state i.e., not till it has been studied under cir- 

 cumstances which, at least approximately, correspond with its 

 natural condition. For in examining stained objects we always 

 have to deal with a dead organism and with altered circumstances 

 which only permit us to form a conditional opinion of the state of 

 things which existed during the life of that organism. 



In some small flasks of beef-bouillon, which forms an excellent 



nourishment for bacteria, a cloudy turbidity of the liquid indicates 



that there are plenty of bacteria in it. A growth of certain bacteria 



on the surface of slices of boiled potato appears as a whitish-gray 



3 



