132 A MANUAL OF BACTERIOLOGY 



thin a layer as desired. The preparation is kept in a moist chamber 

 to prevent evaporation, and can be studied when required. 



The Microscope 



A bacteriological microscope is generally of the mono- 

 cular form, and should be provided with a rack-and-pinion 

 coarse adjustment and an efficient fine adjustment. The 

 stage, preferably of vulcanite, should be large and roomy 

 and quite plain, with two or more holes at its margin to 

 receive spring clips for fixing the slide. For the ordinary 

 examination of specimens a mechanical stage is not needed ; 

 in fact it hampers that freedom of manipulation which 

 is so useful for the rapid examination of a specimen. For 

 some purposes a mechanical stage is very useful, and for a 

 critical survey of the whole of a specimen, e.g. a blood- 

 film, it is essential. A detachable form is to be preferred 

 (Fig. 23), so that, if required, the stage may be free for 

 the examination of plate cultivations, etc. 



New forms of binocular microscopes have recently been 

 introduced by Messrs. Beck and by Messrs. Leitz which 

 possess marked advantages over the monocular instru- 

 ment. 



A sub- stage condenser is essential for all work in which 

 high powers are employed, and also enhances the value of 

 low powers. It consists of a system of lenses below the 

 stage, by means of which the light is concentrated on 

 the object. It should have a rack-and-pinion, or a screw, 

 adjustment for focussing, and be provided with some 

 form of diaphragm for modifying the light, preferably 

 an " iris." The condenser must be centred that is, 

 adjusted so that its optical axis corresponds with the 

 optical axis of the objective ; and for this purpose it ought 

 to be provided with two lateral screws working at right 

 angles to each other, by means of which its position 



