The Microscope. 63 



CORRESPONDENCE AND QUERIES. 



Editor Microscope: 



By the courtesy of James W. Queen & Co., of Philadelphia, 

 who recently sent me for examination a Zeiss Apochromatic yV N. A. 

 1.40, with a full set of eye-pieces, made from the new optical glass, 

 the members of our State Society have had the opportunity of test- 

 ing its claims to superiority. By oblique light it is a well-cor- 

 rected objective, but, in my judgment, no better than our first-class 

 American objectives, except that the images have hardly any percep- 

 tible color. With axial illumination, however, using an Abbe con- 

 denser of N. A. 1.40, with no stops or diaphragms whatever, the real 

 superiority of the glass becomes apparent. I have never before seen 

 so clear and perfect a picture under similar conditions; and it is 

 clearly apparent that the corrections are approximately perfect up 

 to the extreme limit of its aperture. It is not difficult with such 

 axial illumination to resolve a Moller Probe-Platte from end to end, 

 and the images are practically colorless. In the present state of our 

 knowledge, this objective certainly leaves nothing to be desired. 

 The working distance is large, about y^^j inch, and the so-called 

 ^'searcher eye-pieces" make even as high a power as a ^V ^^^7 ^'^^- 

 venient in use. I do not assume to speak for anyone but myself; but 

 such, as it seems to me, must be the judgment of any unbiassed 

 observer. For the practical worker with axial illumination, it seems 

 to me that the Apochromatic objective is destined to become the 

 objective of the future. Yours truly, M. D. Ewell. 



Chicago, January 8, 1887. 



S. D. B. — A good method for keeping chloroform-balsam soft 

 under the cover-glass, is to mount in the ordinary way, set aside 

 until the exuded balsam is covered with a delicate film, then spin a 

 ring of gelatin or gum acacia. Over this, after hardening, (which 

 will take but a few minutes,) apply any permanent ring you may 

 desire. If applied to a fresh mount, the rubber cement, of which 

 you speak, would be dissolved by the chloroform. The same can be 

 said of carbon bisulphide. For delicate specimens, however, glycer- 

 ine IS to be preferred to Canada balsam. 



Fr. D., Melvin, 111. — Inquires: Can skins of insects be kept in 

 alcohol for a month or six weeks without over-hardening ? Yes. 



G. S. A. — A good definition of "numerical aperture" can be 

 found in the Encyclopaedia Brittanica, ninth edition, page 267, 

 under title ''Microscoj)e." 



