USE AND MANIPULATION OF MICEOSCOPE 161 



iris- diaphragm. It is perhaps unfortunate that the majority 

 of cheap microscopes, particularly of the Continental pattern, 

 are not supplied with any arrangement for centreing the sub- 

 stage condenser. High-power work, therefore, if of even an 

 approximately critical nature, becomes somewhat difficult when 

 such an instrument is used. Various types of sub-stage con- 

 densers can be obtained and have already been described, so 

 that it only remains to indicate their methods of use. 



The aperture of the condenser adopted should approximate 

 to, if not equal, that of the objective of highest aperture in 

 use. It will usually be necessary to close the iris- diaphragm 

 to some extent to obtain satisfactory illumination ; but unless 

 the condenser-aperture nearly equals that of the objective, 

 the full advantage of the latter cannot be obtained when the 

 resolution of difficult diatoms or similar objects is attempted. 

 Most achromatic condensers may be used alternatively with 

 medium or high powers if the top hemispherical lens is re- 

 moved, as it usually can be ; the remaining portion then forms 

 a very satisfactory condenser, which can be used with medium 

 powers. It is obvious that, to utilise the whole of the aperture 

 of immersion-objectives, the sub-stage condenser must itself 

 be an immersion one ; but some of these also work fairly 

 satisfactorily if used dry. 



In illuminating an object the first point is to ensure that 

 the optical centre of the condenser coincides exactly with 

 that of the objective. This may be determined by closing 

 the iris- diaphragm as much as possible so that only a very 

 narrow beam of light passes through. The objective should 

 of course be upon the microscope, and some simple object 

 in focus. Back down the condenser until the image of the 

 aperture in the closed iris-diaphragm comes into view and is 

 sharply focussed ; it will then appear more or less well centred 

 in the field of view. By the aid of the centreing-screws on the 

 sub-stage bring this image of the diaphragm exactly into the 

 centre of the field. It is advisable to slowly open the diaphragm 

 until this image reaches the edge of the field. It should be so 

 adjusted that it disappears from view evenly all round the 

 visible field. It is essential that the illuminant should be 

 approximately centred before the adjustment is made, as 

 otherwise it is not easy to get the whole of the outline of the 



