DARK GROUND ILLUMINATION 187 



bright spot it produces on the object is as small as possible, 

 this bright spot is then centred with the lowest-power 

 eyepiece in position. The Nelson condenser has a small 

 ring engraved on the centre of its top lens which can be 

 focussed and centred. 



The most convenient objectives are J", J", or fa", and 

 apochromats show to great advantage over achromats. 

 Care must be taken to correct for cover-glass thickness, 

 and, as any departure from the best adjustment causes a de- 

 terioration in the image , it is well to use the lower-power dry 

 objectives, which are not so sensitive ; magnification can 

 be made up by eyepiecing. The iV' oil-immersion, with 

 its aperture cut down to 1*0 by means of a conical stop 

 screwed inside the mount, combined with a low eye- 

 piece, is also suitable, and is generally the best achromat 

 to use when apochromats are not available. 



As errors of correction in the objective are made rather 

 more apparent with dark ground than with ordinary 

 illumination, it is well to use monochromatic light with 

 achromatic objectives whenever possible. The photo- 

 graphy of moving bacteria under these conditions is, 

 however, impossible unless a very powerful illuminant 

 is employed to reduce the exposure. 



Air bubbles in the immersion fluid must be avoided, 

 as they effectually cut off the light, and the mount must 

 be as thin as possible, as any particles not quite in focus 

 are very annoying. There must also be no dust or 

 scratches on the cover-glass to show up as bright 

 specks and lines. Care must finally be taken to fill the 

 back of the condenser completely with light, as any 

 restriction of the hollow cone that it gives will very much 

 reduce the brightness of the image. 



Cinematomicrography. Dr. Comandon, in conjunction 

 with Messrs. Pa the Freres, has applied dark ground 

 illumination to the cinematography of living and moving 

 micro-organisms and blood preparations. By the courtesy 



