DARK-GROUND ILLUMINATION 167 



object in the field is illuminated. It is likely that then 

 the illuminated disc is not quite central, and it must be 

 centred by the centring screws of the sub-stage ; once 

 the preliminary adjustment of the light has been effected, 

 it must not afterwards be altered. Having thus centred 

 the light with the low-power objective, the latter may be 

 replaced with the higher power objective, which is to be 

 used for the examination, again, if necessary, centring 

 the light with the screws of the sub-stage, and a higher 

 power ocular may then be substituted if required. If an 

 oil-immersion objective be used, the stop for reducing 

 the N.A. must not be forgotten. 



The slides and cover-glasses used must be free from 

 dust, scratches and other blemishes, and be absolutely 

 clean, and the mount must be free from air bubbles. 



The slides must have a thickness of between 1 mm. 

 and 1-2 mm. ; the makers usually specify what thickness 

 of slide should be used with the particular dark-ground 

 illuminator, and it is well worth having a simple micro- 

 meter gauge in order to calibrate both slides and cpver- 

 glasses. With oil-immersion objectives, cover-glass thick- 

 ness does not much matter so long as the objective will 

 focus through it. The proper tube-length should be used 

 with all objectives. In the case of dry lenses thickness of 

 cover-glass must also be corrected for. The thickness 

 usually allowed for is 0-15 to 0-18 mm. If the thickness is 

 greater, tube-length must be reduced ; if less, the tube 

 must be lengthened. If the cover-glass is not of the correct 

 thickness or the tube-length is inaccurate, the image has a 

 very definite haze surrounding it which does not disappear 

 even when the object is accurately focussed. The remedy 

 is to adjust the tube-length until the image appears bright 

 and clear and without any surrounding nebulosity. 



Two oculars should be available, a low-power ( x 4) for 

 adjusting and centring the illuminant and searching the 



