The most perfect method of securing pleasing satisfactory stereoscopic vision of objects is that devised by Mr. Wenham. In his binocular microscope an equal division of the cone of rays, after passing through the objective is secured and again united in the eye-pieces, which act as one, so that each eye is furnished with an appropriate and simultaneous view of the object. The methods contrived by the earlier experimenters not only materially interfered with the definition of the objective and object, but also required expensive alterations and adaptations of the microscope, and sometimes separate stands for their employment. Mr. Wenham’s invention, on the contrary, offers no such obstacle to its use, and the utility of the microscope as a _monocular_ is in no way impaired either when using the higher powers.
The most important improvement, then, effected by Wenham consists in the splitting up or dividing the pencil of rays proceeding from the objective by the interposition of a prism of the form shown in Fig. 43. This is placed in the body or tube of the microscope so as to interrupt only one-half (_a c_) of the pencil, the other half (_a b_) proceeding continuously to the field-glass, eye-piece, of the principal body. The interrupted half of the pencil on its entrance into the prism is subjected to very slight refraction, since its axial ray is perpendicular to the surface it meets. Within, the prism is subjected to two reflections at _b_ and _c_, which send it forth again obliquely on the line _b_ towards the eye-piece of the secondary body, to the left-hand side of the figure; and since at its emergence its axial ray is again perpendicular to the surface of the glass, it suffers no further refraction on passing out of the prism than on entering. By this arrangement, the image sent to the right eye is formed by rays which have passed through the left half of the objective; whilst the image sent to the left eye is formed by rays which have passed through the right half, and which have been subjective to two reflections within the prism, and passing through two surfaces of glass. The prism is held by the ends only on the sides of a small brass drawer, so that all the four polished surfaces are accessible, and should slide in so far that its edge may just reach the central line of the objective, and be drawn back against a stop, so as to clear the aperture of the same.
