74 BACTERIOLOGY. 



may be free from any attachments when required for the examina- 

 tion of cultures. 



Diaphragms are necessary for regulating the amount of light. 

 The plan of using a series of discs, with apertures of different sizes, 

 should be avoided, as they are easily lost, and bacteriological investi- 

 gations may have to be made under conditions in which it is difficult 

 to replace them. A better plan is a revolving plate with apertures 

 of different sizes, but the most convenient form is the iris diaphragm 

 (Fig. 19). 



The sub-stage condenser is quite as necessary in bacteriological 

 work as a high-power objective. In fact, the condenser and the 

 objective should be considered as forming one optical apparatus, and 



the microscope regarded quite as 

 incomplete without a condenser as 

 'it would be without an objective. 



By means of the sub- stage con- 

 denser (Fig. 20) the rays of light 

 are concentrated at one point or 

 on one particular bacterium ; and 

 for the best definition it is essential 

 that there should be mechanical 



FIG. 19,-lRis DIAPHRAGM. arrangements for accurately cen- 



tring and focussing the condenser. 

 It may even with advantage be provided with a fine adjustment. 



To sum up, a microscope for bacteriological investigation should 

 be provided with (1) a steady stand of either the tripod or horse- 

 shoe form ; (2) a tubular body on the Jackson model ; (3) a wide- 

 angled sub-stage condenser, such as Abbe's; (4) objectives of an inch, 

 ^th of an inch, and a yVfch homogeneous immersion ; (5) a removable 

 mechanical stage ; and for the most accurate work there should be 

 centring arrangements and a coarse and fine adjustment to an oil- 

 immersion sub-stage condenser such as Powell and Lea-land's, and 

 a j\th homogeneous oil-immersion apo- chromatic objective. 



With regard to the choice of a microscope, it is chiefly a 

 question of price. The most perfect instrument is the large model 

 by Powell and Lealand, but it is most expensive, and quite unsuit- 

 able for laboratory work. For general use excellent instruments 

 are made by Zeiss, Leitz, Reichert, or Swift. The bacteriological 

 microscopes of these makers are in the necessary equipment 

 practically identical. The Zeiss microscope is the most finished, and 

 costs about twenty pounds. A similar microscope by Leitz and 

 by Swift costs about eighteen, and both make an excellent students' 



