SIR ROBERT HADFIELD, BART. 19 



of the Iron and Steel Institute Journal, 1902, comprising some twenty- 

 three pages in its Glossary of Terms. 



In addition to the main Societies, who have assisted in developing 

 Microscopy, have been the following : The Sorby Scientific Society, 

 comprising The Sheffield Microscopical Society, and The Sheffield 

 Naturalist's Club, which were amalgamated on January 1st, 1918 ; 

 the Quekett Microscopical Club ; the Dublin Microscopical Club, and 

 the Photomicrographic Society. 



Special reference may be made to the excellent work of the 

 Quekett Club, which is probably the most active Microscopical 

 Club in any Country. Its Headquarters are in London, and Meetings 

 are held from time to time. The present occupant of the Presi- 

 dential Chair is Dr. A. B. Rendle, M.A., F.R.S. 



SECTION III.— MODERN WORK ON MICROSCOPES, 

 OBJECTIVES AND EYE-PIECES. 



Mr. Conrad Beck, F.R.M.S., many years ago did valuable work 

 on behalf of Microscopy in his Cantor Lectures before the Royal Society 

 of x4.rts, 1907, on " The Theory of the Microscope." Previous to these 

 Lectures, Mr. John Maynall, junr., gave Mvo excellent series of Lectures 

 on the same subject, entitled " The History of the Microscope," before 

 the same Society. 



An able Address was read by Air. Joseph E. Barnard, now 

 President of the Royal Microscopical Society, in February, 1919, 

 on " The Limitations of Microscopy." Everyone interested in this 

 subject should read the Address, which is divided into various 

 subjects, dealing with dimensions met with in Microscopy, a discussion 

 on the resolving power and limits of resolution and visibility ; also 

 descriptions of the Ultra-Microscope and of experiment sillustrating 

 its use, together with a discussion of the advantages of ultra-violet 

 light in ordinary Microscopy ; and finally suggestions as to future 

 lines of Research. 



As this paper points out, the limit of resolution may be said to 

 have been reached when it is not possible to distinguish the details of 

 the specimen under examination. The limit of visibility is, however, 

 lower than this, for, although no detail can be seen, the specimen can 

 be made visible as a spot in the field of view. 



The question of Resolution is touched upon, from which it appears 

 that under the most favourable circumstances, the practical limit is 

 reached when objects in a row are about "20 micron (1/50,000 cm.) 

 apart. If the body is less than this size under the best microscopic 

 conditions now available no detail can be distinguished. 



The Ultra-Microscope shows the presence of much smaller dimensions 

 than those mentioned above, that is, as bright specks on a dark 

 background, but it shows none of the internal features, and no matter 

 what the shape or nature of the object under view, it always appears 

 circular. The smallest particle observable, that is, in the Ultra- 

 Microscope, is that of colloidal gold, about 5 micromilHmeters 

 (1/2,000,000 cm.) in diameter. Thus the Ultra-Microscope can dis- 

 tinguish particles about forty times smaller than those which can be 

 resolved under the ordinary Microscope. 



