Fig. 41.
Diagram 3, [Fig. 41], represents the method employed by Mr. Wenham for bringing the two eyes sufficiently close to each other to enable them both to see through the double eye-piece at the same moment. a a a are rays converging from the field lens of the eye-piece; after passing the eye-lens b, if not intercepted, they would come to a focus at c; but they are arrested by the inclined surfaces, d d, of two solid glass prisms. From the refraction of the under incident surface of the prisms, the focus of the eye-piece becomes elongated, and falls within the substance of the glass at e. The rays then diverge, and after being reflected by the second inclined surface f, emerge from the upper side of the prism, when their course is rendered still more divergent, as shown by the figure. The reflecting angle given to the prisms is 47½°, to accommodate which it is necessary to grind away the contact edges of the prisms, as represented, otherwise they prevent the extreme margins of the reflecting surfaces from coming into operation, which are seldom made quite perfect.
Fig. 42.—Professor Abbe’s Stereoscopic Eye-pieces.
[Fig. 42] represents a sectional view of Abbe’s stereoscopic eye-pieces, which consist of three prisms of crown glass, a, b and b′, placed below the field-glass of the two eye-pieces; the tube c is slipped into the tube or body like an ordinary eye-piece. The two prisms a and b are united so as to form a thick plate with parallel sides, inclined to the axis at an angle of 38·5°. The cone of rays from the objective is thus divided into two parts, one being transmitted and the other reflected; that transmitted passing through a b and forming an image of the object in the axial eye-piece B. Adjustment for different distances between the eyes is effected by the screw placed to the right-hand side of the figure, which moves the eye-piece B′, together with the prism b′, in a parallel direction. The tubes can also be drawn out, if greater separation is required. The special feature of this instrument is that on halving the cone of rays by turning the caps, an orthoscopic or pseudoscopic effect is produced. This double-eyed piece arrangement of Abbe’s has not been at all brought into use in this country; this is partly owing to its original adaptation for use with the shorter Continental body-tube of 160 mm., and not for our 10-inch body.
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.
Fig. 43.
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.
