970 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tion of images by the passage of light through small apertures, and 

 conceives that Dr. Norris's " colourless disks " are merely images at 

 the end of the microscope-tube or the aperture of the eye-piece, and 

 he seems to have taken some pains to obtain such images by placing 

 under the Microscope a slide thickly strewn with small steel disks, 

 and receiving the light on a screen beyond the eye-piece. Had he 

 attempted to focus these ghosts and the real images of the disks at the 

 same time, or considered a little more closely the elementary optical 

 principles involved, we venture to say the note would never have been 

 written. 



The ToUes-Wenham Aperture Controversy. — The address* of 

 Dr. J. D. Cox, the President of the American Society of Micro- 

 gcopists, is exclusively occupied with a review of the controversy 

 between Mr. Wenham and Mr. Tolles on the aperture question, with 

 extracts from the various papers published by them and others. Mr. 

 Wenham was so fundamentally in the wrong throughout that con- 

 troversy, not only on the merits of the question, but also in the 

 manner in which his part of the controversy was conducted, that Dr. 

 Cox may be, in part at least, forgiven for the relentless manner in 

 which he recapitulates the strange optical errors which Mr. Wenham 

 from time to time enunciated, not omitting the mishap by which — 

 though in fact he had discovered, in 1855,| the great increase of 

 distinctness of the more difficult diatoms when mounted in balsam and 

 with a small hemisphere cemented over them with balsam — he after 

 all missed the keystone of the aperture question and the important 

 property of immersion objectives in consequence of having in some 

 inexplicable way supposed that a glass hemisphere did not magnify 

 the object at its centre because there was no refraction.j 



If, however. Dr. Cox, in demonstrating the correctness of the views 

 of the American optician, felt himself obliged to deal so fully with 

 the mistakes of his opponent, he does not shrink from paying a well- 

 deserved tribute to Mr. Wenham in the following words : — " His 

 authority was deservedly great. His improvements of the Microscope 



* Proc. 7th Ann. Meeting Amer. Soc. Micr., 1884, pp. 5-89 (4 figs.). 



t Quart. Jonrn. Micr. Sci., ill. (1855) p. 302. 



X Ibid., and Mon. Micr. Journ., x. (1873) pp. 11-12. The text of Mr. "Wen- 

 ham's paper is as follows : — 



" Now arose the question of a means of obtaining the full aperture on objects 

 in balsam or fluid. It at once appeared that if the object was set in the centre 

 of a sphere (or hemispliere) all rays from the central point must continue 

 their course without deviation, and that in such a case neither the length of radius 

 of the glass hemisphere or the refractive power of the material would influence 

 the results. I therefore made a number of minute plano-convex lenses of various 

 radii, some less than the 1/lOOth part of an inch. Such of these as turned out to 

 he hemispheres were set exactly over a single selected diatom and balsam let in. 

 Before the balsam was admitted for a well-known optical law, the object could 

 not be seen. "When a l/5th or other object-glass was brought over this lens, the 

 arrangement might be termed a four-system one, though the optical efi"ect of the 

 hemisphere as a lens was nil, simply because there was no refraction. The 

 balsam object was not magnified. It occupied a like focal distance to the dry 

 ones outdde and the same adjustment served for eitiier." Cf. also Mon. Micr. 

 Journ., ix. (1873) p. 31. 



