150 A MANUAL OF BACTERIOLOGY 



must be centred that is, adjusted so that its optical axis 

 corresponds with the optical axis of the objective and 

 for this purpose the sub -stage fitting ought to be provided 

 with two lateral screws working at right angles to each 

 other, by means of which the position of the condenser 

 relative to the optical axis can be altered (see p. 161). 

 In order to obtain maximum resolution with lenses of 

 high numerical aperture it is necessary to employ an 

 oil-immersion condenser of similar numerical aperture. 

 With such a condenser a large drop of cedar oil is placed 

 on the top lens of the condenser, which is then racked up 

 until the oil touches the slide carrying the object, care 

 being taken that no air bubbles are included. Con- 

 densers for dark-ground illumination also work in oil. 



Below the sub -stage condenser a mirror with concave 

 and plane surfaces should be fitted, the plane surface 

 being used with the condenser, as a general rule. The 

 concave mirror may occasionally be used for illumination 

 with low-power objectives, the condenser being detached 

 or swung out of position. Careful illumination must be 

 insisted upon and the condenser must be carefully f ocussed ; 

 to obtain the best results the light should be readjusted 

 for every specimen, i.e. correct or so-called " critical " 

 illumination should be aimed at. Not only may a good 

 specimen be spoilt, visually, by faulty illumination, but 

 a true image of the object and the full resolving power of 

 the objective are obtainable only by critical illumination. 

 In the examination of micro-organisms in the fresh or 

 living and unstained condition, it is necessary, as a rule, 

 to diminish the light by means of a small diaphragm, or 

 by racking down the condenser, or by both ; while for 

 stained or opaque objects the full aperture of the dia- 

 phragm, or thereabouts, may generally be employed. It 

 must be remembered, however, that the resolving power 

 of a lens (see below) is diminished by dosing the diaphragm 



