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of sufficient value or importance to bring it into use, and therefore Wenham’s instrument is scarcely likely to be superseded.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.] CHAPTER II. 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. [Illustration: Fig. 47.--Visual Angle.]