208 E. M. NELSON ON DARK-GROUND ILLUMINATION. 



needed fully 300 mm. of tube before the field became velvety 

 black. Microscopists should therefore be on the look-out for 

 this sort of thing. 



With regard to all these new kinds, or rather old kinds 

 revived, of paraboloids and what not, it was my opinion, when 

 formerly they w^ere in vogue, that none of them could com- 

 pare with an oil-immersion condenser, and that remains my 

 opinion still. 



There are many objectives which perform well enough on 

 objects on a bright ground, but on a dark ground the mist 

 remains, adjust the tube -length how you may. The only thing 

 to be done is to put a stop at the back lens and cut down 

 the aperture. This, I understand, is now often done. 



I regret not being able to show you the apparatus myself, but 

 you will not lose anything on that account, as Mr. Curties, who 

 made both the new condenser and bull's eye, will kindly exhibit 

 them for me. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. XL, No. 68, A^i~il 1911. 



