Royal Microscojpical Society. 165 



III. — Note on the Resolution of Amphifleura pellucida hy certain 

 Objectives made hy R. and J. Beck, and hy William Wales. 



By Dr. J. J. Woodward, U. S. Army. 



(Bead before the Royal Microscopical Society, March 6, 1872.) 



In the 'Monthly Microscopical Journal' for Sept., 1871, p. 150, 

 I recorded the resolution of this difficult diatom by an immersion 

 ^th made by Mr. Tolles, of Boston, and sent photographs exhi- 

 biting the performance, to the Editor, as well as to the Eoyal 

 Microscopical Society. 



The same desire to do full justice to all honest efforts to improve 

 the microscope, which led me to make that brief jDublication, induces 

 me now to give like publicity to the performance of the new 

 immersion ^^th (so-called) of Messrs. E. and J. Beck. 



In the first instance, I thought it worthy of note that, by 

 excellent workmanshij) and high angle, a lens of such comparatively 

 low magnifying jjower could achieve so great a feat of resolution. 

 In the second instance, the objective having more than double the 

 magnifying power, the result is only surprising on account of 

 the extremely low price at which the makers have undertaken to 

 furnish it. 



]Mr. Jos. Beck exhibited during his visit to America an immer- 

 sion y\jth of excellent quality. One was ordered for the Army 

 Medical JMuseum, which has recently come to hand, and it is of its 

 performance I now speak. These glasses are remarkably cheap as 

 compared with any similar ones hitherto brought to my notice. 



The objective received at the Museum is characterized by great 

 flatness of field, freedom from colour, and dej^th of penetration. Its 

 performance on Amj^hij^leura pellucida may be fairly judged from 

 the accompanying photograph,* which is magnified 1025 diameters. 

 On the Nobert's nineteen-band plate I get it satisfactorily through 

 ih.Q sixteenth band. As to its magnifying power and equivalent 

 focal length, I obtained the following results : — With 50 inches 

 distance from micrometer to screen the magnifying power was 570 

 diameters at uncovered, and 630 at full correction for cover. 

 Taking uncovered as a standard, therefore, the focal length should 

 be rated at about ^^^h inch. The angle of aperture at covered is 

 about 160^. 



A few days after taking the photograph of Ainpliipleura 

 pellucida above referred to, I made another photograph of the same 

 diatom with an objective made for the Museum two years ago by 

 Mr. Wm. Wales, of Fort Lee, New Jersey. This is the objective 



* Photographs were received from the author, and fully bear out what he 

 states. — Ed. • M. M. J.' 



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