MICROSCOPY. 



499 



"While much remains that is at present in- 

 volved in obscurity, it seems probable, from 

 the earnest attention that is now being directed 

 to the study of micro-organisms, that discov- 

 eries will be made in this field that will revo- 

 lutionize alt our previous ideas with regard to 

 the origin and treatment of disease. 



MICROSCOPY. Before discussing the progress 

 recently made in the evolution of the com- 

 pound microscope, in which the enlarged im- 

 age, formed by one lens or system of lenses, 

 called the objective, is viewed through another 

 magnifying system called the ocular, it will be 

 profitable to consider briefly the present con- 

 dition of the simple microscope, in which a 

 single amplification is produced by a simple 

 lens, or by a system acting as such. 



Notwithstanding the growing use of low- 

 power objectives in the study of large objects, 

 or of large and serial sections, and the employ- 

 ment of objectives as erectors, giving a very 

 low range of powers, and the introduction of 

 excellent compound binocular dissecting mi- 

 croscopes, the simple microscope is still indis- 

 pensable in both field and laboratory work. 



The Simple Microscope. For the low powers 

 of the simple microscope, non-achromatic 

 lenses are still employed ; and by some makers 

 they are so mounted as to be used either sin- 

 gly or in pairs. A useful special form, whose 

 trumpet-shaped tube furnishes an efficient 



FIG. 1. WATCH-MAKER'S GLASS, WITH SPRING HOLDER. 



shade to protect the eye from extraneous light, 

 is the " watch-maker's glass," which, of various 

 powers and with one or two lenses, is always 

 attainable, and is often mounted upon an ex- 

 temporized stand by those fond of amateur 

 arrangements, or is added to an existing simple 

 microscope to increase the range of its outfit. 



In the dissecting microscope of C. Ye 1 rick, of 

 Paris, an identical arrangement is adopted for 

 the regular supply of lenses.* For very low 

 powers, however, the u engraver's glass," a 

 doublet of great size mounted in hard rubber, 

 is preferable. It has long been used by a few 

 microscopists, for such work in the selection, 

 study, manipulation, or dissection of objects as 

 can be carried on under a very low amplifying 

 power; but the merits of its broad field, clear 

 definition, and ease to the eyes, are only now 

 becoming generally realized. It requires a 

 large, heavy stand, like the "lens-holder," Fig. 

 3, which was not easily available hitherto, but 

 is now made for the purpose. For higher 

 powers, as from one to one fourth inch, the 

 Coddington is still the best of cheap and non- 

 achromatic magnifiers, and is supplied on dis- 

 secting or, as better named on the Continent, 

 preparing microscopes. The small n on- achro- 

 matic doublets and triplets, formerly import- 

 ant, and planned with great care, are now 

 wholly superseded in their field by achromatic 

 combinations. 



A few simple microscopes are arranged to 

 use medium and low-power achromatic object- 

 ives from the compound microscope, which, 

 notwithstanding the excellence of their correc- 

 tions, and the advantage of being always at 

 hand, are only tolerably convenient for the 

 purpose. 



The " globe lens " of Gundlach is an achro- 

 matic Coddington, being a sphere of crown- 

 glass, balsam-cemented into the center of a 

 hollow sphere of flint-glass, reduced to a cylin- 

 drical form, as in the plain Coddington, by 

 cutting away the unused peripheral portion. 

 Being achromatic, the blackened diaphragm 

 groove is not required. This arrangement se- 

 cures, in addition to the well-known advan- 

 tages of the Coddington, a clear and brilliant 

 definition at the center of the rather limited 

 field. 



By other makers solid three-lens achromatic 

 magnifiers are produced, under the names of 

 " aplanatic loupes," "platyscopic lenses," or 

 "achromatic triplets." These were early 

 brought into service in America by Tolles and 

 Zentmayer; the most common form, perhaps, 

 consisting of a thick lens of crown-glass like a 

 shortened Coddington, achromatized by a thin 

 concavo-convex meniscus of flint-glass at each 

 end. These triplets, usually mounted as shown 

 in Fig. 2. give an exquisite definition, a broad 



* The ordinary watch-maker's glass, if used without a stand, 

 and supported by catching- its flaring- tube in the recess of the 

 eye. becomes tiresome and disagreeable in prolonged use. 

 The form shown in Fig. 1. however, which was introduced in 

 1880 by the Bausch & Lomb Optical Co., and which is auto- 

 matically supported by a steel spring passing around the ob- 

 server's head, may be worn with comfort, and seems to be 

 the most practical magnifier which, for its focusing, depends 

 upon the steadiness of the head. It can readily be raised 

 out of the way, and rested upon the forehead during occa- 

 sional use of the eyes for ordinary vision ; and when again re- 

 quired it can be instantly restored to position over the eye by 

 a touch of the hand. It is likely to prove more available 

 as a dissecting microscope than the magnifiers heretofore 

 mounted in spectacle-frames for similar purposes. 



