[Illustration: Fig. 45.--Swift-Stephenson’s Erecting Binocular.]
No variation or change of any kind proposed either in the form of the instrument or the prism has proved of sufficient value or importance to bring it into use, and therefore Wenham’s instrument is scarcely likely to be superseded. It must be admitted that the improvement effected in the eye-piece form by Mr. Tolles, of Boston, U.S., is an exception to the rule laid down. It consists in mounting the prisms in a light material, vulcanite, made to fit into the monocular microscope body, thus taking the place of the ordinary eye-piece. The image transmitted by the objective is brought to a focus on the face of the first equilateral triangular prism by the intervention of an erector-eye-piece inserted beneath it. The second set of prisms have a rack and pinion movement to adjust them to any visual angle. The illumination of both fields in this eye-piece is nearly equal in brightness. Mr. Stephenson’s erecting binocular (Fig. 45) has proved to be of some practical value. It has the advantage of being of equal use with high and low powers, and with little loss of definition. When used for dissecting purposes it gives an erect image of the object. It is equally useful as a working microscope, for arranging diatoms and botanical specimens of every kind. The sub-stage tube will receive a diaphragm or illuminating apparatus; the eye-pieces have a sliding adjustment for regulating the widths between eyes.
[Illustration: Fig. 46.--An early form of the Ross-Wenham Binocular; nose-piece and prism-holder detached.]
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.
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]