578 Transactions of the Society. 



this cannot be decentred ; and lastly, the objective remains avail- 

 able also for observation in the ordinary way without dark-ground 

 illumination ; one may even say that it must give better images, 

 than it did before the central portion had been stopped out, because 

 according to the laws of the diffraction theory, a diffraction disc 

 produced by an annular opening of suitable dimensions is even 

 somewhat smaller than that formed by the full aperture under 

 similar conditions. 



An alteration has been made in the mount of the condenser 

 which enables a convenient and rapid change to be effected from 

 an optical system of 1 ■ 4 N.A., as used for the usual illumination 

 of bacteria (after Dr. Koch), to another optical system consisting 

 of an objective specially corrected and stopped down to a small 

 aperture by which a dark-ground illumination is obtained, and 

 which allows sunlight or arc light to be directly employed. 



On the table here I have a preparation of ordinary cholera 

 bacillus shown by this method. It will be noticed how thick and 

 pronounced the appearance of the exceedingly fine flagella has 

 become. But it is not this to which I would draw your attention so 

 much as to a number of bright discs representing something which 

 lies in the same thin layer on which the objective is focussed,, 

 but which cannot or can scarcely be seen by ordinary methods 

 of observation. Ultra-microscopic bacteria might be expected to 

 look something like this, though of course, I do not intend to 

 suggest that there are any in the preparation on view. 



In conclusion I must point out that these investigations have 

 been materially assisted by the liberal manner in which all the- 

 necessary means were placed at our disposal by the firm Carl 

 Zeiss of Jena. 



