The Microscope. Its History, Construction, and Application 15th ed. / Being a familiar introduction to the use of the instrument, and the study of microscopical science

by Jabez Hogg

Simple and Compound Microscope.

Microscopes are known as simple and compound. The simple microscope may, for convenience, be divided into two classes; those used in the hand (hand magnifiers), and those provided with a stand (mounted, as it is termed) for supporting the object to be viewed, together with an adjustment for the magnifying power, and a mirror for reflecting the light through the object.

Fig. 47.—Visual Angle.

A simple microscope, mounted, is preferable to a single lens, being usually composed of two or more lenses separated by a small distance on a common axis; the increase of the size of an object being the angle it subtends to the eye of the observer, or the angle formed by the combination drawn from the axis of vision to the extremity of the object, as in [Fig. 47]. The lines drawn from the eye to a and r form an angle, which, when the distance is small, is nearly twice as large as the angle from the eye to o w, formed by lines drawn at twice the distance. This is called the angle of vision, or the visual angle. Now, the utility of a convex lens interposed between a near object and the eye consists in its reducing the divergence of the rays forming the several pencils issuing from it, so that they enter the eye in a state of moderate divergence, as if they were issuing from an object beyond the near point of distinct vision, and a well-defined image is thereby formed upon the retina. In the next [Fig. (48)], a double-convex lens illustrates the action of the simple microscope, the small arrow being the object brought under view, and the large arrow the magnified image. The rays having first passed through the lens are bent into nearly parallel lines, or pencils diverging from some point within the limits of distinct vision. Thus altered, the eye receives rays precisely as if they had emanated directly from a larger arrow placed about ten inches away from it. The difference between the real and the imaginary object represents the magnifying power of the lens. The object in this case is magnified nearly in the proportion the focal distance of the lens bears to the distance of the object when viewed by the unassisted eye; and this is due to the object being more distinctly viewed so much nearer to the eye than it otherwise could be without the lens.[18]

Fig. 48.—Virtual Image formed by Convex Lens.

It should be remembered that the shorter the focus and the nearer the eye the magnifying lens is placed the smaller will be the diameter of the sphere of which it forms a part, and unless its aperture be proportionally reduced, the distinctness of the image will be destroyed by the spherical and chromatic aberrations of its high curvature. Nevertheless, it was by the use of lenses so constructed that the older microscopists—of whom Leeuwenhoek was the more eminent—were enabled to do so much excellent work.

The various kinds of simple pocket lenses for the most part consist of a double-convex, or a plano-convex, or a combination of both, varying in focal length from a quarter of an inch to two inches. Sometimes they are set in pairs with a hole, a small diaphragm, cut in the piece of horn placed between them. These are extremely useful for carrying in the waistcoat pocket; to the anatomist and field botanist for examining various objects and preparations.

Fig. 49.—Wollaston’s Doublet.

Perhaps the most important improvement effected in this form of the simple microscope was that ascribed to the celebrated Dr. Wollaston, who devised a doublet of two plano-convex lenses having their focal lengths, in the proportion of one to three, mounted with their convex side directed towards the eye of the observer, and the lens of shorter focal length next the object. The explanation given of the correction thus effected in Dr. Wollaston’s doublet will be best understood on reference to the annexed diagram, l l′, in [Fig. 49], being the object for a segment of the cornea of the eye, and d d′ the stop or diaphragm. Now, it will be seen that each pencil of light proceeding from l l′, the object, is rendered excentrical by the limiting aperture or the diaphragm d d; consequently, they pass through the lenses on opposite sides of their common axis o p; thus each becomes affected by opposite errors, which to some extent balance and correct each other. To take the pencil l, for instance, as it enters the eye at r b; r b is bent to the right at the first lens, and to the left at the second; and as each bending alters the direction of the blue ray more than the red, and as the blue ray falls nearer the margin of the second lens, where the refraction is greater than that nearer the centre, and compensates to some extent for the greater focal length of the second lens, the blue rays will emerge very nearly parallel, and colourless to the eye. At the same time, its spherical aberration has been diminished, since the side of the pencil as it proceeds through one lens passes nearer the axis, and in the other nearer the margin.

This must be taken to apply to pencils farthest from the centre of the object. Central rays, it is obvious, would pass both lenses symmetrically, the same portions of rays occupying nearly the same relative places in both lenses. The blue ray would enter the second lens nearer its axis than the red; and being thus less refracted than the red by the second lens, some amount of compensation would take place, differing in principle, and inferior in degree, to that which is found in the excentrical pencils. In the intermediate spaces the corrections are still more imperfect and uncertain; and this explains the cause of aberrations which must of necessity exist even in the best-made doublet. It is, however, infinitely superior to a single lens, and will transmit a pencil of an angle of from 35° to 50°.

The next step towards improving the simple microscope was in relation to the eye-piece, and was effected by Holland. It consisted in substituting two lenses for the first in the doublet, and placing a stop between them and the third. The first bending of the pencils of light being effected by two lenses instead of one, produces less spherical and chromatic aberration, which are more nearly balanced or corrected at the second bending, and in the opposite direction, by the third lens.

Another form of simple lens was devised by Dr. Wollaston, the “Periscopic.” This combination consists of two hemispherical lenses cemented together by their plane faces, with a stop between them to limit the aperture. A similar proposal, made by Sir David Brewster in 1820, is known as the Coddington lens,[19] shown at [Fig. 50]: this has a somewhat larger field, and is equally balanced in all directions, as is made evident, the pencils a b and b a passing through under precisely the same circumstances. Its spherical form has the further advantage of rendering the position in which it is held of comparatively little consequence. It is still used as a hand magnifier, although its definition is certainly not so good as that of a well-made doublet. It is usually set in a folding case, as represented in the figure, and so contrived as to be admirably adapted for the waistcoat-pocket. It is usually sold with the small holder, [Fig. 50]a, for holding and securing small objects during examination. Browning’s Platyscopic Pocket Lens is a useful form of pocket lens for the botanist and mineralogist. Its focus is nearly three times longer than that of the Coddington, and allows of opaque objects being more easily examined; it has also a magnifying power of 15, 20, and 30 diameters.

Fig. 50.—The Coddington Lens.

Fig. 50a.

Fig. 51.—Steinheil’s Aplanatic Lens.

One of the best combinations of the hand or pocket form of lens is that known as Steinheil’s aplanatic lens ([Fig. 51]); it consists of a bi-convex lens cemented between two concavo-convex lenses, giving a relatively long focal distance and a large flat field. The higher powers of this lens are much used for dissecting purposes. This handy magnifier appears to have suggested a later combination, the apochromatic of Zeiss. No hand lens can compare with Steinheil’s “loups.”

Fig. 52.—Simple Microscope.

When the magnifying power of a lens is considerable, or when its focal length is short, or it is wished to use it with greater precision and steadiness, it should be mounted on a short stand with a tubular stem, with rack-work focussing movement and mirror illumination. [Fig. 52] represents a simple dissecting microscope, with a glass circular stage, 4½ inches in diameter, supported on three legs—a handy and useful form of instrument for many purposes.