Nearly all objectives from the one inch upwards of any considerable aperture give images of the object seen from a different point of view with the two opposite extremes of the margin of the cone of rays; the resulting effect is that there are a number of dissimilar perspectives of the object blended together at one and the same time on the retina. For this reason, if the object under view possesses bulk, a more accurate image will be obtained by reducing the aperture of the objective.
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 47-1/2°, 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.
[Illustration: 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.