148 A MANUAL OF BACTERIOLOGY 



moist chamber to prevent evaporation, and can be studied when 

 required (Delepine). 



The Microscope 



A bacteriological microscope is generally of the mono- 

 cular form, as the ordinary binocular is unsuitable for 

 use with high powers and with short tube length, and 

 should be provided with a rack-and-pinion coarse adjust- 

 ment and an efficient fine adjustment. The stage, pre- 

 ferably 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 examina- 

 tion of specimens a mechanical stage is not needed, but 

 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. If a mechanical stage be fitted, 

 the surface should be flat, and any bar, stops or clips 

 removable ; failing this a detachable form is to be pre- 

 ferred, so that, if required, the stage may be free for the 

 examination of plate cultivations, etc. 



Special forms of binocular microscopes have been intro- 

 duced by Messrs. Beck and by Messrs. Leitz which possess 

 marked advantages over the monocular instrument for 

 some forms of work. 



The short or Continental tube of 160-170 mm. is 

 generally adopted, being a more convenient length to 

 work with than the English tube of 250 mm. The longer 

 the tube the greater the magnification, but objectives 

 are corrected for a definite tube-length and should be used 

 only for the length for which they are corrected. An 

 inner extension or draw-tube, preferably with a rack and 

 pinion, should be provided so that the long tube-length 

 may be used if a lens is corrected for it. The draw-tube, 

 moreover, permits of two corrections to be made : (1) if 

 a nose-piece or objective changer is used the tube-length 

 is obviously so much increased and, provided the outer 



