THE microscope (from ?????, little, and s??p??, to see), so called because it enables us to see objects which are too small to be seen with the naked eye, consists of several parts, each of which has its special use. As the proper management of these is of great importance in the successful application of the instrument to minute investigations, we shall commence with the consideration of their names and uses, including those of the more important pieces of accessory apparatus.

Microscope.—The foot of the microscope is that part which supports the instrument upon the table; it is connected above with the stand, of which it is often considered a part. The stand sometimes consists of a single rod or pillar; but in the best microscopes it is composed of two upright plates, between which, at the upper part, the rest of the microscope swings stiffly upon an axle. Arising from this axle, indirectly through the medium of parts which require no special mention, is an arm, to which the body is fixed. The body is moveable up and down by one or two large milled heads, connected with a grooved rod or pinion, which works in the teeth of a rack fixed to the back of the body, or of the arm which supports the body. The large milled heads form the “coarse movement,” as it is called.

On the top of the arm, or on the front and lower part of the body of the microscope, is placed the “fine movement,” consisting of a small milled head, with a fine screw, for moving the body through very small distances.

Next is the “stage,” or flat plate, upon which the objects to be viewed are placed. This is often so arranged that, by turning two milled heads, the object can be moved backwards and forwards, or from side to side; it is then a “moveable stage.”

The eye-piece slides into the upper end of the body; and the object-glass screws into its lower end.

Beneath the stage is the mirror, which reflects the light through the object, the object-glass, and eye-piece to the eye.

Object-glasses.—The object-glasses are the most valuable parts of the instrument. There are generally three or more of them; and, by means of an “adapter,” any object-glass can be made to fit any microscope. Great care is required in their use, especially to avoid scratching the lower surface of the glass, which is sometimes accidentally done by pressing the surface against any hard body, or allowing such a body to fall upon it. When not in use, the object-glasses should either be put away in the brass boxes or covered with a small bell-glass, to prevent their receiving any injury.

The object-glasses possess various magnifying powers, according to the distance at which they require to be placed from the object for distinct vision: this is not, however, absolutely correct, yet may serve as a general expression. Thus we have a 1-inch, ½-inch, ¼-inch object-glass, &c. The object-glasses, for brevity, are often called powers.

As a beginner may at first have some difficulty in distinguishing a high from a low power, it may be remarked that the size of the lower glass is larger the lower the power: but, in the case of the better object-glasses, the focal distance is engraved on the box in which the object-glass is packed when put away.

As the coarse movement raises or depresses the body and object-glass through comparatively large distances, it must be used only with the lower object-glasses, i. e. those of low or little magnifying power, as the 2-inch, 1-inch, or ½-inch, or to bring the object-glass near the focal distance with the higher powers; whereas the fine movement serves to adjust the higher powers, as the ¼-inch, &c.

If the object-glasses should become soiled on the lower face, this should be wiped very gently with an old silk handkerchief or piece of very soft wash-leather, previously shaken to displace dust. The same method will answer to cleanse the upper surface of the eye-piece.

Great care must be taken that a slide which has been warmed in any experiment be not placed near the object-glass until quite cold.

Mirror.—The mirror has sometimes one silvered face only, at others two—one flat, the other concave. The flat surface is used to reflect the light upon the object when the light is too great with the concave surface.

Beneath the stage, in most microscopes, is a circular moveable “diaphragm,” perforated with holes of various sizes, to allow more or less of the light reflected by the mirror to pass through, as may be required.

When opake objects are viewed, the mirror should be turned aside, so as not to reflect any light through the stage.

Eye-pieces.—With all microscopes, two or more eye-pieces are supplied. These possess different magnifying powers, and are lettered or numbered accordingly; the lowest power with the earliest letters of the alphabet, or with the smallest numbers, thus: A, B, C; or 1, 2, 3, &c.

Forceps.—These are fine pincers, for holding minute bodies to be viewed as opake objects. In use, they are inserted by a stem connected with a joint, in an aperture, generally in the stage; and are moveable in all directions.

Live-box.—This is a brass slide, perforated in the middle, to the aperture in which is soldered a short piece of brass tube, closed at the top with a circular plate of thin glass. A rather wider and longer piece of brass tube slides over the former; this is also closed at the top by a thin glass plate, so as to allow of an object being confined or compressed between the two glass plates. It is used for examining living objects in water.

Knife, &c.—For cutting slices or sections of objects, a very sharp knife with a thin back will be found useful; or a razor may be used for the same purpose. And for picking minute objects to pieces, or dissection, fine needles, cut off short with pliers, the blunt ends being thrust into hair-pencil sticks, will be requisite.

A pair of fine surgical forceps will also be required, for taking up minute objects. These should be without teeth, and the spring-action so weak that the points can be very easily approximated.

Dipping-tubes.—For removing minute objects from water, two or three narrow glass tubes, of different lengths, are very useful. These are called “dipping tubes,” and are used thus:—the tube being held upright between the second finger and the thumb, the fore finger is placed at the top of the tube to close it; the tube is then put into the water until the lower end is close to the object, when, on suddenly removing the fore finger, the water will rise in the tube, carrying the object with it. The fore finger is then again applied to the tube, and, as thus held, the water will not run out. The tube is then held over a watch-glass, or a slide, upon which the water and object will fall on removing the fore finger.

A small glass spirit-lamp will be found very useful. The spirit for burning should be methylated alcohol, or wood-naphtha. As these spirits are inflammable, great care should be taken to keep the stock-bottle away from a candle or other flame, when filling the lamp.

Achromatic condenser.—A very important piece of apparatus, when high powers are used, is the achromatic condenser; it is not, however, usually supplied with the cheaper microscopes. It consists of a brass fitting, placed beneath the stage, into which an object-glass is screwed, in an inverted position, i. e. the small end of the object-glass being placed uppermost. It serves to condense the light to a focus upon the object, so as to illuminate it more brightly; and as it can be elevated or depressed by a milled head and rack-work, the object can be viewed by either converging or diverging rays.

Simple microscope.—For examining the larger kinds of objects, and for dissection, a simple microscope is very useful. This consists of a stand, a stage, and an arm supporting a simple lens or combination of lenses, but without the body of the compound microscope (as the ordinary microscope is distinctively called). For most purposes, common plano-convex or doubly convex lenses are sufficient to form the object-glasses of a simple microscope. With the best microscopes, an “erector,” or tube containing a pair of lenses, fitted within the body, renders the compound microscope capable of answering most of the purposes of a simple microscope.

Polariscope.—An expensive but interesting and useful addition to a microscope is a polarizing apparatus, or polariscope. This consists of a Nicol’s prism, or a plate of tourmaline, placed beneath the stage, and another in the body of the microscope or above the eye-piece; both in brass fittings. The former is called the polarizer, and the latter the analyzer.

Rotating disk.—Another most useful piece of apparatus, for moving opake objects whilst under the microscope, in all directions, is Smith and Beck’s “rotating disk.”

Slides.—The slides upon which objects, especially those to be viewed as transparent objects, are to be placed, should be made of crown or plate glass. They are usually 3 inches long, and 1 inch wide; but I prefer them 2½ inches long, and 1 inch wide, simply because they take up less room in a cabinet, and because they do not project beyond the stage on either side. They should not be more than ¹/₂₀th of an inch thick, and as colourless and clear as possible. The edges should be ground or filed, to prevent their scratching the stage.

Covers.—The covers are square pieces of very thin glass, less in breadth than the slides, so as not to reach their margins; and of various thicknesses, the thicker and stronger being used to cover large objects for examination under the lower powers, and the thinner serving to cover very delicate objects requiring the higher powers.

Side condenser.—For illuminating opake objects, a large plano-convex or doubly convex “bull’s-eye” lens, or side condenser, is used; this is fixed to an arm, which slides on a stand, so as to be capable of being raised or lowered to a suitable height. This is placed between the source of light and the stage, and at such a distance from the latter that the light may be brought to a focus upon the object. Sometimes a “Lieberkuhn” or concave silver reflector is used for this purpose.

These are the most important pieces of apparatus required in examining microscopic bodies. But the beginner will do well, if he have the achromatic condenser and the polarizing apparatus, to lay these aside until he has had considerable practice in examining objects simply with the mirror and the lower powers.

General method of observation.—In the ordinary use of the microscope, the object to be examined is laid upon the middle of a slide, which is placed upon the stage. The object is then brought under the centre of the object-glass, the mirror inclined half towards the light and half towards the object, until the object is seen to be illuminated, when, upon looking through the eye-piece and adjusting the coarse and fine movements, the object as it comes into focus will be seen, as it were, drawn upon a white disk, which is called the “field.”

When the object is wet, it cannot be viewed without the application of a cover, because the water evaporates and condenses upon the under surface of the object-glass.

To avoid the danger of injuring the object-glass, or crushing the object by lowering the body and object-glass too much in adjustment to focus, the best plan is to lower the body by means of the coarse movement until the object-glass appears near the object to the eye placed on one side of the stage, and then to apply the eye to the eye-piece, and turn the milled head so as to raise the body and object-glass until the object is brought into focus.

In the examination of an object, it is best to begin with a low power, so as to obtain a view of the general arrangement of its parts, and then to apply the higher eye-pieces and powers, so that the more minute structural details may be observed.

Illumination.—The illumination or proper management of the light in using the microscope is of very great importance. The best light, especially with the low powers, is daylight, particularly that reflected from white clouds; this is least injurious to the eyes. But as daylight cannot always be used for microscopic investigations, and as cloud-light is insufficient with the higher powers, some kind of artificial light must be supplied. That mostly used is the light of a reading oil lamp or of a gas-burner, a candle being quite useless, on account of the flickering of the flame with the slightest draught. A moderator lamp has the defect of too great height; otherwise this would be better than any other oil lamp. A powerful and excellent light for the highest powers is afforded by a short paraffine-oil lamp.

As intently looking at strongly illuminated objects is injurious to the sight, the amount of light allowed to pass the diaphragm should be no more than is agreeable, and sufficient to show the object distinctly.

In using the higher powers, the field is much less bright with the use of the same light than in the case of the lower powers; and difficulty is often found in obtaining sufficient light for the distinct vision of the object. The achromatic condenser is of most important service here; but it is sometimes requisite, even when this is used, to condense the light upon the mirror by a shallow bull’s-eye; or a large common metallic reflector may be used for the same purpose.

The most convenient manner of proceeding in regard to the use of the individual eyes is to apply the left eye to the eye-piece, so that the right eye may be used in finding the stage-movements, or in moving the slide, without removing the eye from the eye-piece. If this arrangement be adopted, the light should be placed towards the left-hand side of the microscope. But the best way to avoid injuring the sight would be to use both eyes for viewing the objects in turn, although most microscopic observers make use of one eye only for this purpose.

The structure of many transparent objects can be best seen when the mirror is turned more or less obliquely to one side, so as to view them by oblique light, as it is called: we shall refer to this point hereafter.

As a rule, objects are best seen by transmitted light, or as transparent objects, although it is well to examine objects under both kinds of illumination, i. e. by transmitted and reflected light.

If during the use of the microscope, after removing the eye from the instrument, the impression of the light remains perceptible to the sight, the light used has been too strong, or its action too long continued; and the instrument should be at once laid aside for a time.