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V. — The Advancing Powers of Microscopic Definition. 



By Dr. Royston-Pigott, M.A. Cantab., M.E.C.P., F.C.P.S., 

 F.R.A.S., M.R.I., F.R.M.S. 



As it is now above two years since I had the honour of laying before 

 the readers of the Transactions of the Royal Microscopical Society 

 some of the results to which I had been led by several years' patient 

 inquiry, I may now be permitted to state that further investigations 

 have confirmed the views then first broached. During this interval 

 I have had the honour of receiving several communications from 

 distinguished microscopists confirmatory of their truth, and among 

 tJiem I wish to mention that of Colonel Dr. Woodward, who has 

 most kindly forwarded to me tln-ee distinct sets of his peerless 

 photographs. Dr. Woodward has favoured me also with the War 

 D epartment Memorandum on Test Podura, from which I take leave 

 to make a few extracts, as placing the matter in a clearer light 

 th an any words of mine can do. 



" War Depaktment, Surgeon-General's Office, 

 Army Museum, Febniary 22, 1871. 



" For a long time the scales of certain insects have been favourite 

 test-objects for high-power definition. Among these especial import- 

 ance has been attached to the so-called Podura scale, which has been 

 extensively used by English and American microscopists, as well as 

 by makers of objectives, who have employed it to try their glasses 

 during the process of construction. The market was supplied with 

 slides from various sources, which were oifered for sale by all vendors 

 of objects for the microscope, under the general designation of ' Podura 

 scales.' The markings on the scales, as seen by a good l-5th or by 

 higher powers, were generally described as resembling exclamation 

 marks, without the lower dot, and with a clear line of light running 

 down the mark. It was thought that those objectives were most 

 perfectly corrected, which showed these exclamation marks well 

 defined, with the points sharp, the butts handsomely rounded, the 

 contours black, the central line of light brilliant and broadest towards 

 the butt, and the whole scale as free as possible from chromatic 

 eifect." .... 



" Under these circumstances considerable interest was excited 

 among microscopists by the paper of Dr. G. W. Eoyston-Pigott, 

 which appeared in the ' Monthly Microscopical Journal' for December, 

 1869 (communicated May 21, 1869). Dr. Pigott asserted that the 

 exclamation marks were an optical deception, produced by two sets 

 of beads crossing each other at a small angle, each of the exclamation 

 marks corresponding to three or four beads." .... 



" The photographs (of the Lepidocyrius curvicollis) are taken from 

 a slide presented by Mr. Joseph Beck, partly because the scales on 



VOL. VII. F 



