Page:Hardwicke's Science-Gossip - Volume 1.pdf/285
contact with a horse-show magnet attached to the under-surface of the stage, are rendered magnetic by it. A soft iron bar resting upon them at right angles, serves as a ledge for objects to rest against when the microscope is inclined from the vertical position. The attraction between the bar and the tongues is just sufficient to keep the former in its place with any ordinary object that may be resting against it, and is not too strong for being easily shifted about.
As the modern microscope possesses many adjuncts—as condensers, spotted lenses, polarizers, &c., all of which are used beneath the stage; and as these need permanent support and accurate centering, a moveable sub-stage is constructed, into which they all fit. A traversing movement is given to the stage itself, by rack-work and pinion. The microscopist is thus enabled to shift the position of his various apparatus to suit his requirements. Between the stage and sub-stage is a shutter for regulating the amount of light reflected upward from the mirror; it is termed the diaphragm, and consists of a thin circular plate perforated with holes varying in diameter, the largest being equal to the central aperture of the stage, and the smallest not much larger than a pin-hole, while in one position the light is cut off altogether. The plate revolves on a pivot, so that each aperture may, in its turn, be brought under the stage. The diaphragm should never be permanently fixed to the stage, but should be so attached as to be easily removed when other modes of illumination are desired. An improved form of diaphragm has lately been constructed, in which the circular shutter with its various openings has been superseded, and in its stead we have but one aperture, which can be gradually closed by two small plates that move horizontally to the right and left of the stage, and have their proximal borders incised in a wedge-shaped manner. The advantage of this disaphragm consists in the nicety with which the orifice can be regulated so as to obtain the most effective illumination.
Another important part of the microscope is the mirror, by whose aid we collect and reflect the light upwards to the object-glass. This should in all cases be double, having one of its surfaces flat and the other concave, with a diameter not less than an inch and a half, or even more. The greater the extent of the reflecting surface, the larger will be the amount of light obtained therefrom. As so much depends on the proper illumination of an object, the mirror should be so constructed as to admit of free and easy motion in all directions. It should slide freely up and down the stem which supports it, thereby enabling the microscopist to regulate the intensity of its light as he increases or decreases its distance from the stage; and if it be mounted on a lengthening arm, a further advantage will be gained by the oblique illumination it produces.
When high powers are used, a more intense light than that afforded by the mirror is needed. The microscopist then has recourse to the various kinds of condensers in vogue. These are usually achromatic combinations of lenses, and, when properly adjusted, condense on the object in one brilliant spot all the light reflected from the mirror. Our space, however, precludes us from entering further into the subject of illumination.
Having selected a good solid stand, the microscopist should next secure the best lenses that can be procured from our most skilful opticians. These may be purchased one at a time, according to the means and wants of the purchaser. Two of the most useful powers are the one-inch and one-fourth objective; and with these two powers a large amount of useful work may be done. Good object-glasses may be known by the clear and distinct images they give of suitable objects placed in their foci, and a bad objective may be equally well known by the absense of these qualities. No amount of careful focussing will ever enable such a lens to give a clear and well-defined view of an object submitted to its scrutiny. The eye of the observer, however keen, sees nothing distinctly where all is involved in fog.
One very good test of the quality of an object-glass of moderate power is the way in which it bears the strain of the higher eye-pieces, even when the draw-tube is pulled out to some extent. If the combination be properly corrected for light and colour, it will give a flat field, with clear definition in every part of it, and will suffer scarcely any deterioration in its performance. The reverse of this will hold good with inferior lenses. The question of angular aperture scarcely comes within the scope of a short paper; but those who are intersted in this branch of optics will find it fully discussed in our standard works on the microscope. The ordinary method of attaching an object-glass to the body of the microscope is by screwing it into the tube, and as the threads of the screw are generally very fine, some little time is consumed in substituting one lens for another; to obviate this difficulty, and to facilitate a rapid exchange of objectives, a mechanical contrivance has been invented, termed a nose-piece, which consists of two or more rotating arms, each carrying an objective at its free end. By the rotation of the arms, each lens, in its turn, is brought into its proper position beneath the tube.
The eye-pieces (of which some three or four with varying magnifying powers are supplied by our best makers) should be constructed so as to slide gently into the upper end of the body, a mode of attachment that gives great facility to the microscopist for changing his eye-pieces.
One of the greatest modern improvements in connection with the compound microscope undoubtedly is Mr. Wenham's binocular arrangement. Its simplicity and facility of adaptation to almost every kind