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PEOCEEDINGS OF THE SOCIETY. 



AN ORDINARY MEETING 



OF THE Society was held at 20 Hanover Square, W., on 

 Wednesday, March 20th, 1918, at 5.30 p.m., Mr. J. E. 

 Barnard, President, in the Chair. 



The Minutes of the last Meeting, having been circulated, were taken 

 as read, confirmed, and signed by the President. 



The President reviewed the purpose of the Meeting, and, after 

 welcoming Professor Conrady, requested him to address the Meeting on 

 " The Theory of Dark-ground Illumination." 



• Professor Conrady's address, which was freely illustrated by sketches 

 and diagrams on the blackboard, may be briefly summarized as follows : — 



Objects lying in a perfectly transparent medium are seen on a dark 

 ground when illuminated exclusively by rays at a greater obliquity than 

 the objective can receive, because the only light which can then enter 

 the instrument is that which has been scattered by the objects. 



Some objects only scatter the light in directions close to that of the 

 illuminating rays ; hence the gap between the latter and the most 

 oblique rays which the objective can receive should be as small as 

 possible. Other objects scatter the light in distinct, widely-separated 

 beams ; to accommodate these, very oblique illuminating rays are also 

 necessary. 



The scattering may be in any direction ; the entrance of light into 

 the instrument therefore depends upon uniform distribution of the 

 illumination with reference to the axis of the instrument — in other words, 

 a perfectly centred hollow cone of rays is essential. 



The realization of these conditions by a condenser with dark-ground 

 stop depends on the condition that the stop is depicted in the limiting 

 aperture of the objective. This condition is not even approximately 

 fulfilled by the condensers of high N.A. and short focal length, hence 

 the bad effects obtained and the call for reflecting condensers. 



The theory of microscopic resolution under dark-ground illumina- 

 tion is simple only in the case of regular structures treated on Abbe's 

 principle. It then leads to the conclusion that the condenser should 

 have three times the N.A. of the objective, and that full resolving power 

 is therefore only obtainable with objectives up to 0*35 or 0*45 N.A. 



