The improvements which have been effected in the formation of the object-glasses of refracting microscopes have rendered these so very superior to the reflecting microscopes, that the latter have fallen into disuse.

Conditions of Efficiency.-These are the same as those necessary for the perfection of natural vision, viz., 1st, sufficient visual angle; 2nd, sufficient distinctness of image; and 3rd, sufficient illumination.

The greater the visual angle, the more perfect is the distinctness of the image, both as respects form and colour, provided the aberrations, spherical, chromatic, and astigmatic, are corrected by the material and form of the lenses, as explained when treating of photographic lenses, or objectives. The illumination will depend upon the intensity of the light used, and the angular aperture of the object-glass.

Smith and Beck's Popular Microscope.-The construction of this microscope will be readily understood by referring to Fig. 77. The body A is carried by a strong arm B, which is attached to a square bar C, that may be moved up or down by a rack-work and pinion in the lower part of the stand, when the stage D, and the mirror E, are attached.

The base F is triangular, and connected with the parts of the instrument already described by a broad stay G, which moves on centres at the top and bottom, so as to allow the end of the tube H to fit by its projecting pin into various holes along the medial line of the base. With this arrangement, if the body of the microscope be required in a more or less inclined position, as in Fig. 77, four holes are provided near the extremity of the base for the pin of the tube to fit into. A hole near the stout pin L is used when a vertical

position is wanted; while to obtain the horizontal posi

tion the pin of the tube is placed in a hole in the stud

N

K, the inner surface of the stay G resting at the same time on the top of the stout pin L. This form of construction is entirely new, and possesses the following advantages:-It is strong, firm, and yet light; the instrument cannot alter from any particular inclination it is put into, which is not unfrequently the case when the ordinary joint works loose; and in every position the heavier part of the stand is brought over the centre of the base, to insure an equality of balance.

Directions for Use.-To adjust the focus of the objectglass, turn the milled heads o for a quick movement, or the milled head P for a slow one. The stage D is circular, and upon it fits a plate T; this again carries the object-holder u, which is provided with a ledge v, and a light spring w; it is held on the plate т by a spring underneath, so that it can be moved about easily by one or both hands. The small spring w is fastened to the object-holder by a milled head, which will unscrew; so that the position of the spring may be altered, to give more or less pressure upon the edge of the object, or it may be removed altogether, if neces

sary.

When a stage with only a flat surface is required, the object-holder u may be removed by unscrewing from the under-side of the plate T two small milled heads, which connect a circular spring with the objectholder; or, by removing the plain stage altogether, an extra simple flat plate may be substituted.

Beneath the stage there is a cylindrical fitting, for the reception of a diaphragm, or for any additional apparatus that may be required in that position.

The mirror E, besides swinging in a rotating semicircle, will slide up or down the tube н, or it will turn on either side for oblique illumination.

The light should in general be on the left of the observer. The best is that from a white cloud on a bright day; but a satisfactory effect can be obtained from a wax or Palmer's candle, if protected by a glass, a Cambridge or moderator oil-lamp, a small paraffin or belmontine lamp, or an Argand gas-burner, provided it is not more than ten or twelve inches from the instrument.

The management of the illumination demands particular attention. That of a transparent object is produced by reflection from the mirror below, which should have its centre coincident with the axis of the body, and should be at such a distance that the light reflected from it may nearly converge to a focus at the object. This distance will be about two-and-a-half inches when daylight is used; but the rays from a lamp or candle, ten or twelve inches from the mirror, are so divergent, that the focus for them will be about three inches, and the mirror may have to be slid up or down accordingly.

Accurate adjustment of this focus is often required with the quarter-inch object-glass; and some details of objects, such as delicate striæ, are best seen with this glass when a strong light is thrown on them obliquely by turning the mirror on one side of the axis. With the one-inch object-glass the light is generally in excess, and has to be lessened by fitting the diaphragm under the stage. This admits only so much light as passes through one or the other of the two apertures in a small revolving disc; by which contrivance, together with sliding the diaphragm up more or less under the stage, every necessary variation can be made.

To illuminate opaque objects the light is thrown upon them from above by a small condensing lens,

mounted upon a separate stand, and capable of being turned in any direction; its focus for a lamp or candle four inches from it is about three inches; for daylight two inches. A large object can be placed upon the stage at once; but small ones are either laid on a piece of glass or held in a forceps supplied with the instrument; they fit upon the pin at the top of the small milled head, which fastens the spring on the stage; and by the ball-and-socket movement at a, and the sliding wire b, every requisite movement can be obtained. In illuminating objects from above, all light that could enter the object-glass from below should be excluded; this can be done effectually by turning over the aperture the blank space of the diaphragm.

Wenham's Binocular Body.—Thus far in this description the microscope has only been considered as having a single body; the addition, therefore, of the binocular body, shown in section in Fig. 78, requires a few explanations and directions for use. The purpose of the binocular microscope is to give stereoscopic vision of objects, whereby the form, distance, and position of the various parts are instantly seen; and the result is almost as striking as if the minutest object were placed in the hand as a model.

To accomplish this, the only plan hitherto known is the equal division of the rays after they have passed through the object-glass, so that the eye may be furnished with an appropriate one-sided view of the object; but the methods hitherto contrived to effect this not only materially injure the definition of the object-glasses, but also require expensive alterations in their adaptation, or, more frequently still, a separate stand; whereas the following arrangement, contrived by Mr. Wenham, is no obstacle to the use of the