To illustrate this, Fig. 40 _a_, _b_ are enlarged drawings of a portion of the egg of the common bed-bug (_Cimex lecticularis_), the operculum which should cover the opening having been forced off at the time the young was hatched. The figures exactly represent the two positions that the inclined orifice will occupy when the right- and left-hand portions of the object-glass are stopped off. This object is viewed as an opaque object, and drawn under a two-thirds object-glass of about 28° aperture. If this experiment is repeated, by holding the card over the eye-piece, and stopping off alternately the right and left half of the ultimate emergent pencil, exactly the same changes and appearances will be observed in the object under view. The two different images just produced are such as are required for obtaining stereoscopic vision. It is therefore evident that if instead of bringing them confusedly together into one eye we can separate them so as to bring together _a_, _b_ into the left and right eye, in the combined effect of the two projections we obtain at once all that is necessary to enable us to form a correct judgment of the solidity and distance of the several parts of the object. 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. [Illustration: 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 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.]