PEOGRESS OF MICROSCOPICAL SCIENCE. 33 



On the Illumination of Binocular Microscopes, by Dr. E. H. Ward, 

 of Troy, suggested convenient means of regulating illumination in the 

 naturalist's every-day work with the microscope, and lu'ged that pro- 

 fessional microscopists make their influence more distinctly felt in 

 regard to the lower classes of instrmnents that are furnished to be- 

 ginners, and particularly in regard to popularizing the Binocular 

 Microscope. 



Photographs, by Dr. Madclox, of tlie Podura *S'caZe. — President 

 Barnard, in exhibiting these, gave an exhaustive review of the dis- 

 cussion in regard to the structure of the scale. The traditional " note 

 of exclamation," or goose-quill markings, are unlike those of any other 

 known scale, and many naturalists are anxious, on groimds of analogy, 

 to get rid of them, Mr. Beck argued that these marks represented 

 parallel lines on different sides of the scale, crossing each other at an 

 acute angle, and necessarily imperfectly focussed ; some observers have 

 attributed them to corrugations or folded ridges of the upper and 

 lower membranes of the scale ; and Dr. Pigott, with his aplanatic 

 searcher, and others have seemed to resolve them into bead-like rows 

 of spherules, between two membranes. The use of reflected light to 

 determine these points is very desirable, but difficult with sufficiently 

 high powers. Professor Smith, of Kenyon College, proposed to make 

 the objective its own illuminator. Others have replaced the mirror he 

 placed behind the lenses by a plate of glass or a prism ; but all these 

 means give a glare of light by reflexion from the surfaces of the lenses. 

 The speaker had proposed a concave mirror behind the outer pair, an 

 internal Lieberkuhn, which works exceedingly well with medium 

 powers, say one-third or one-fourth inch ; but there is not room for its 

 insertion in high powers. As compared with Tolles' prism, which is 

 similarly situated (above the front pair), it gives more light, and 

 illuminates from any part or all parts of the circmnference at will ; on 

 the other hand it is less easily apj)lied, requiring the front lens to be 

 mounted in glass instead of brass, and it is inapjilicable to large opaque 

 objects. The beaded api^earance has not yet been satisfactorily seen 

 by reflected light ; nor is it well shown in the photographs where the 

 wedge-shaped dashes seemed rather marked by cross-lines or partial 

 interruptions. The speaker evidently doubted the accuracy of the 

 exclamation points, but was not yet ready to accept the beads. Ap- 

 pearances best seen by pushing an objective far beyond its ordinary 

 power were received with "general distrust. In the discussion which 

 followed the reading of this paper, Dr. Ward remarked that the pro- 

 duction of a beaded appearance, as a purely optical effect, should be 

 considered no longer doubtful, but rather an occasional accident to 

 persons using high powers. As an extreme instance, in the case of 

 a coarse and familiar structiu'e, he related that while experimenting 

 upon an elater of Marchantia polymorpha, that beautiful double spiral 

 "was " resolved " into three rows of " beads " or " hemispheres," perfectly 

 distinct and immistakable, which occuj)ied, of course, the position of 

 the middle and edges of the spiral. They were illiuninated by parallel 

 light, very oblique under a y'^^th objective of 175° worked at a power 

 of 3000 diameters. 



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