E. M. NELSON ON DARK-GROUND ILLUMINATION. 207" 



have a small centring spot engraved upon the surface of its front 

 lens so that the condenser may be centred to the optic axis by 

 means of a low power. 



I have to-day seen, for the first time, the flagellum of the- 

 tubercle bacillus with a dry lens upon a dark ground. A few- 

 months after Dr. Koch's discovery of the tubercle bacillus I 

 recommended this plan of viewing it upon a dark ground to the- 

 medical profession. It was exhibited both to this Club and at the 

 Koyal Microscopical Society.* Further, I photographed it upon 

 a dark ground and exhibited the photomicrograph upon the 

 screen ; but the medical profession said that dark grounds were 

 not used in Germany, so they would have none of them. Now 

 that dark grounds are used in Germany they cannot have enough I 



With my new oil-immersion condenser the light obtained in 

 this manner with a |-in.-wick paraffin lamp (the same one 

 exhibited here scores of times in years gone by) gives ample 

 illumination for observing the tubercle bacillus on a dark ground 

 with a dry |^-in, and a x 42 -power eye-piece. The condenser will 

 work with slips of "8 to 1*2 mm. thick. 



A thick diatom, such as Eupodiscus argus or Aulacodiscus,. 

 when illuminated upon a dark ground, was so brilliant with a 

 5-power eye-piece that I could not stand the light. 



In conclusion, with regard to the quality of a dark ground, an 

 object should appear brilliantly illuminated and as it were lying 

 upon a piece of black velvet. There should be no mist or 

 cloudy nebulosity about either the image or the field. If there 

 is, the fault may lie either in the substage condenser or in the 

 objective. It will be found that the correction of some object- 

 glasses may differ very considerably with objects upon a 

 dark ground ; for example, a sharp, brilliant lens of about 

 ^-in. focus and about 0*9 NA., which upon an object on a 

 bright ground would require a tube-length of 160-180 mm., 



* Journ. Q,M,S., Ser. 2 Vol. 1, p. 380, and Journ. Ji.M.8., Ser. 2, Vol. 4, 

 p. 497 (1S84). 



