2GG

MICROSCOPES, BY BOSS, SMITH AND BECK, VARLEY, Ac.

[Class X.

neral purposes of natural history or anatomical investi-gation.

Combinations of high power, and extremely extendedangles of aperture, are excellent in developing one classof test objects, viz., minute lines or dots on plane sur-faces, and admirably demonstrate the high perfection towhich such glasses are capable of being carried byscientific opticians; but such combinations, with a lessangle of aperture and more penetrating power, arc farmore generally useful and valuable to the minute anato-mist and the naturalist.

In regard to the brass-work, we may observe that thequalities especially requisite in the stand of a microscopeare simplicity of construction, portability, combined withsufficient weight to ensure safety and steadiness, withsmoothness and accuracy of action in all the workingparts, and such a construction as to distribute any tremorthat mav be communicated to the instrument equallyover its* body, stage, and other working parts. Thesedesirable points are admirably attained in the form sug-gested by Mr. George Jackson, and adopted by 'Messrs.Smith and Beck, Boss, and other makers. For purposesof delineation, Nachet’s ('France , No. 1370) form ofprism is more advisable thau that of Wollaston’s, as theformer having one reflection less than the latter, presentsthe image to the eye in an erect instead of an invertedposition.

We now proceed to discuss the particulars of eachinstrument.

Boss (No. 254, pp. 435, 430) exhibits a microscope, themechanical parts of which are exceedingly good: themovements are very smooth and true; the stand is on aplan which is solid and steady, and at the same time notcumbrous. The object-glasses are constructed with dif-ferent kinds of glass in the different compound lenses,forming a combination so as to double up the secondaryspectrum, and this is done so well that scarcely anyseparation of colours can be detected. The angularapertures of the object-glasses examined are as follows:—1-inch focal length, 27 3 aperture,i-inch „ 60°

I-i»eh ,, 113 d ,,

{-inch ,, 107' ,,

^-inch ,, 135° ,,

Both the half-inch and the one-eighth of an inch fociare purposely made of smaller proportionate aperture thanthe quarter-inch, or the one-twelfih of an inch, as in alllenses of large aperture, the image becomes indistinctfrom the slightest change of focus:—and so, unless anobject be an absolute plane, it is impossible to see thewhole field tolerably distinct at once with an object-glassof large aperture. In the set examined, the inch, thehalf-inch, and the one-eighth of an inch, are intended forthe general examination of objects; and the one-fourthand one-twelfth of an inch for the examination of minutestructures. The object-glasses are first-rate. A CouncilMedal was awarded to Mr. Ross.

Smith and Bkck. (No. 253, p. 435) exhibit a micro-scope, the stand of which in ajipearanee is not highlyfinished, but their forbearance to expend time and moneyon elaborately finishing the non-working part has beenadopted on the strong recommendation of some of the !oldest naturalists in London , in order that students may Iacquire instruments with first-rate glasses at the least ;possible expense, and that such instruments may be 1brought within the compass of those whose means are 1limited. The stand is excellent in principle: the body, istage, and appliances beneath are all carried on one stout <cast bar, on the recommendation of Mr. E. Jackson, by |means of which the centring of the achromatic illumi- ■nation is rendered easy and certain, and on any tremor ;being communicated to the instrument, it is equally dis- :tribnted over the whole of the working parts.

The lever motion to the stage of this instrument is themost easy and generally useful that has yet been applied.If used with the right hand, while the quick and slowadjustments to the focus are worked with the left, thereis no animalcule that cannot be readily followed, howeverfitful and rapid its movements; and any globule of blood

pursuing its course through the most tortuous of thecapillaries, can be steadily and easily traced, and everyalteration of its form observed during its passage throughthese minute vessels. The field of view may also beswept horizontally or perpendicularly, and the mostdelicate micrometrical measurements made with greatease and precision. This stage is the invention of Mr.Alfred White; the rabbited groove on which the bodymoves was suggested by Mr. George Jackson, at whoserecommendation the fulcrum of the stage movement wasfixed to a spring, instead of to a rigid bar. The sim-plicity and efficiency of the whole of this stand are highlycommendable. The object-glasses examined were offirst-rate quality, and were as follows:—

§-inch focus of 45 ; aperture,j^-inch „ 70“ to 75- aperture.

^,-inch ,, GO 3 aperture.

I-inch ,, 100' to 105° aperture.

They are beautifully corrected for spherical aberration,but the secondary speculum has not been much diminished.The half-inch focus of 70 aperture is a wonderfully fine! combination, easily showing objects, considered difficultj for a one-eighth inch focal length a little more than a; year since, and bearing the application of the higher eye-pieces in an unprecedented maimer.

Smith and Beck also exhibit all that is necessary forthe mounting of microscopic objects, as cells, slips, thinglass, fluids, covers, Ac., and a few preparations as speci-mens. There is, also, a new form of cabinet, for thereception of objects, the names of which may be exposed,by means of porcelain labels with which they are fur-nished, and from which the pencil-writing can be easilyeffaced.

There are two tables with revolving tops, by whichthe microscope can be turned readily round for the con-venience of examination by different observers, and thusrendered a social instrument. The microscopes are fur-nished with portable silver reflector ami annular con-denser, which exhibit transparent objects upon a darkground. (This invention was made by Mr. Wenham, andSmith and Beck claim its first execution.) A CouncilMedal was awarded to Messrs. Smith and Beck.

Vari.ky and Sox (No. 257, p. 430) exhibit a micro-scope, the stage of which is moved by parallel rods, withball and socket joints, which arrangement gives auequable motion in all directions, and is specially adaptedfor the examination of living objects. A second micro-scope is exhibited, adapted for receiving vials in whichaquatic plants or animalcule may be kept in a living statefor any length of time. The plant is secured to one sideof the vial by a piece of cork, and thus is within thereach of the microscope. The vial is kept full of water,and is only corked when used, at which time it is held ina jacket to cut off all extraneous light; a dark chamberprojects from it, opposite to the magnifier, so that a singlebeam of light may be made to fall upon the part underexamination.® A third microscope, of a simple construc-tion, is also exhibited, chiefly intended for beginners.

Yarley’s lever stage is very much more complex thanWhite’s; and as the lever is placed behind the stage it isless convenient to use.

King (No. 287, p. 439) exhibits a microscope stand,with micrometers and goniometers. It has a pyramidaltripod, with stage traversed in rectangular planes bymicrometer screws. The parts of this instrument are soarranged that its weight is equally distributed over thebase, and when inclined at its working angle, the prin-cipal weight is below the point of suspension, and thestand is steady and good. The traversing-stage is fur-nished with divided scales and verniers. The workman-ship throughout this instrument is of the first order. Itis furnished with many ingenious applications of sub-sidiary instruments, and of apparatus specially adaptedto the examination of objects by polarized light, andgoniometric apparatus for measuring the angles of micro-scopic crystals. The mode of illumination, by a prism

* This instrument is fully described in the Transactionsof the Society of Arts.”