THE 



MONTHLY MICROSCOPICAL JOURNAL. 



APEIL 1, 1871. 



I. — On the Structure of the Podura Scale and certain other Test- 

 ohjects, and of their Rej)resentation hy Photo-micrography. 



By Lieiit.-Col. Dr. J. J. Woodward, U. S. Army. 



(^Eead lefore the Eoyal Mickoscopical Society, March 1, 1871.) 

 Plates LXXIX. and LXXX. 

 The ' Monthly Microscopical Journal ' for December, 1869, contains 

 a paper " On High-power Definition," by Dr. Gr. W. Eoyston- 

 Pigott. This essay was rapidly followed by others on allied sub- 

 jects, the whole constituting a remarkable series. They contain 

 mathematical discussions of many points connected with the optical 

 questions involved in the use of objectives, and especially of immer- 

 sion objectives ; assert the existence of " a certain residuary aberra- 

 tion, chiefly spherical," in the best modern objectives ; * claim the 

 correction of this aberration, partly by special modes of illumina- 

 tion, but chiefly by means of a vaguely-described optical combina- 

 tion, " the Aplanatic Searcher ;" f and finally, contain many interest- 

 ing and some novel statements with regard to the structure of 

 several test-objects, and especially the Podura scale. 



These papers have provoked considerable discussion- in England, 

 and perhaps a few temperate remarks on the subject by a Trans- 

 atlantic observer will not be without interest. 



My observations will relate chiefly to the structure of the 

 Podura scale and a few other test-objects, and to the possibility of 

 representing these objects photographically. 



The mathematical portions of Dr. Pigott's papers must be left 

 to such appreciation as the mathematicians may award ; but his 

 general assertion that our finest objectives are not perfectly cor- 

 rected, is doubtless substantially true. It is well known that the 

 lenses sent out by the best makers continue annually to improve ; 

 and I, for one, should be sorry to believe that we have yet reached 

 the limit of perfectibihty. How far the " Aplanatic Searcher " can 

 be regarded as a valuable addition to the optical appliances at the 

 disposal of the microscopist, is a question on which, perhaps, any 



* 'Monthly Microscopical Journal,' vol. ii., p. 296. t I^'-. vol. iv., p. 42. 



VOL. V. M 



