MICROSCOPE, an optical instrument by which images of objects are so magnified that details invisible or indistinct to the naked eye are clearly observed. In the ordinary micro scope the magnifying power is interposed di rectly between the eye and the object, in the manner of a magnifying glass; and although the power may consist of several lenses, they combine as one. See LENS.
gives the first magnified image is the one nearest the object, and therefore called object glass or objective. The optical part which mag nifies the image formed by the objective is the one to which the eye is applied and is called the eye-piece or ocular. This latter, in its common Microscopes are of two types—simple and compound, the former being used for low mag nifications, rarely exceeding 20 diameters, whereas the latter may give as high as 4,000 diameters magnification. In the simple form the eye views the object directly (Fig. 1), whereas in the compound form an enlarged image is formed by one lens, which image is magnified by another lens or pair of lenses, at the same time reversing it so that what is at the right hand in the object is at the left hand in the image. (Fig. 2.) A short focus posi tive lens becomes a simple microscope when used for directly viewing an object; its usual form is that of the Pocket Magnifier, and al though generally consisting of a combination of two or more lenses with the view of im proving its quality, it always remains optically simple. The most simple forms are one or several convex lenses mounted separately and offering a variety of foci, and hence of magni fying power. These lenses have two defects, one, chromatic aberration, which fringes the images with the colors of the spectrum, most noticeably red or and blue; the other, spherical aberration, which is most noticeable by .the lack of distinctness increasing toward the edge of the field. Improved forms are constructed to overcome these defects. The most simple of these is the Coddington lens, originally a section of a sphere, but, as generally made, a thick double convex lens with a circu lar groove which acts as a diaphragm. The achromatic lens when composed of three lenses, two concavo-convex flint glass lenses enclos ing and cemented to a double convex crown glass lens, is the best form. These are usually placed in a folding mounting for pocket use. The simple microscope is also made in more complex form for dissecting purposes, a me chanical construction, more or less elaborate, being added, which provides adjustment for the lens in relation to the object, a platform or stage for the latter, and a mirror for re flecting light to illuminate the object. (Fig. 3).
In the compound microscope the lens which form, is called a Huyghenian, eye-piece, after Huyghens, who used it in the earliest and most primitive telescope construction. It con sists of two plano-convex lenses (Fig. 2), suit ably mounted in brass, the lower lens, il, being called the field lens, the upper one, op, the eye lens. It is a negative eye-piece, as its focus lies within itself, a diaphragm cd which limits the field of vision being placed at this point. The action of the compound microscope is plainly shown in Fig. 2. The image of the object ab is carried through the objective gh to the field lens i/ of the eye-piece, and thence to a focus at the diaphragm cd. At this point it is viewed by the eye lens of the eye-piece and magnified to the size ef. Fig. 4 shows the appearance of the working instrument in which the principles exhibited in the diagram, Fig. 2, are applied. The particular instrument shown is fitted with three objectives of different focus set on a pivot, so that any one of them may be brought into the axis of the tube. There is a rack and pinion for adjusting the focus and micrometric attachments for moving the object on the stage. The tube and stage are pivoted on the two pillars to incline at any convenient angle. The magnification of the compound microscope depends upon three conditions: (1) The power of the object-glass, (2) the power of the eye-piece, (3) the amount of the objective 150 diameters. A convex lens of one inch focus gives a magnification of about 10 diameters at a distance of 10 inches, and this holds true of a combination of lenses of this equivalent focus as in the eye-piece.
If therefore a one-inch focus eye-piece is 10 inches distant from a one-inch objective, the magnifying power is 10X 10=100 diameters; or, if 5 inches distant, is one-half as great, or 50 diameters. The designation of power is according to the focus of a single lens having the same magnifying power as the series or combination of lenses which make up the objective as well as eye-piece. As the image of the objective is magnified by the eye-piece, it is evident that any defect in the objective is magnified to the same extent, and unless elimi nated would seriously interfere with obtaining a distinct image. The main problem, therefore, remains to convey through the objective as many image-giving rays, free from defects, as possible. As a matter of fact, objectives, what ever their power, are composed of a series of lenses whose purpose it is to correct errors which would exist if single lenses alone were used, and the greater the power of the objec separation of these two optical parts. If the focus of the object-glass is reduced, the power is increased, and the same holds true with the eye-piece. The more the objective and eye piece are separated, the greater will be the power. It will appear from this statement, therefore, that the magnification of the micro scope is unlimited, but the mere magnification of an object is less sought after and is of less value in the modern microscope than its defini tion or power to disclose detail and structure. The length of tube which connects the eye piece and objective is limited to from six to eight inches, for the sake of convenience in use. A draw-tube permits the length to be extended to 12 or 13 inches for the highest powers. The standard length of the micro scope tube must be closely adhered to or else the optical capacity (correction) of the objec tive will be disturbed. The power of the eye piece rarely exceeds 15 diameters and that of tive, the larger the number of lenses required. In the low powers there are generally two systems of lenses, each of which is an achro matic doublet ; in the medium powers the prin cipal magnification is obtained by a single front or hemispherical lens and two systems of cemented and corrected lenses; in the high powers it is usual to employ two superposed hemispherical lenses, adding thereto two cor rected combinations. As may be supposed, the production of these lenses and setting them in mountings involves the most accurate processes. First of all the various kinds of glass must have fixed and previously calculated properties and be of absolute homogeneity and freedom from blemishes. The production of such glass is in itself a laborious and delicate process. The lenses must be accurately ground and polished to absolutely correct spherical sur faces, truly centred and cemented, then set in suitable mountings without strain at absolutely correct distance and the axes of all in align ment. The efficiency of an objective to gather up rays emanating from an object and form a perfect image depends upon its angular aper ture — in other words, upon the angle at a point in the axis of the microscope subtended by the diameter of the object in its normal horizontal plane; and this really determines the visibility of detail (correction for chro matic and spherical aberration being presup posed). On account of the loss of light and inability to obtain sufficient angular aperture in the ordinary way in the very high powers, it is necessary to construct them for immersion con tact with the object, and they are then termed 'immersion objectives.* The immersion me dium commonly used is a drop of cedar oil, which has the same refractive index as glass. The connection of the objective with the object by the globule of oil prevents the light rays from scattering. The highest power in general use is the 1/12-inch focus, giving a magnification up to 3,000 diameters, the me dium powers are g to g inch, and the low powers from 1 tos inch.