This difference may in part be attributed to the practical modification in the angle of aperture of the objective, produced by the division of the cone of rays transmitted through the two halves, so that the picture or image received through each half of the objective of 60° is formed by rays diverging at an angle of only about 30°. He confesses, however, that this does not satisfactorily explain the fact that the binocular brings to the _mind’s_ eye the _solid_ image of the object, and thus gives to the observer a good idea of its form and which could hardly be obtained by the monocular microscope. Carpenter cites in support of his views the wing of a little-known moth, _Zenzera Œsculi_, which has an undulating surface, whereon the scales are set at various angles instead of having the usual imbricated arrangement, a good object for demonstrating; the general inequality of surface and the obliquity of its scales, which are at once seen by the binocular with a completeness not obtained by the monocular instrument. To one unaccustomed to work with the binocular the views expressed by Dr. Carpenter as to the extreme value of the instrument for ordinary work may appear somewhat exaggerated, but from my own experience, having long had in constant use a Ross-Zentmayer binocular, furnished with a special prism, constructed for working with a 1/8 dry objective or a 1/10 immersion, the perfection of picture obtained was in every case quite equal to that of the monocular microscope. The relief to the eyes can hardly be over-estimated; the slight inequality of the pencil rays may be regarded rather as a part of the welcome rest afforded when a prolonged examination is made; it certainly appears to me to equalise the slight physiological difference known to exist between the eyes of most people. If one image is seen a little clearer by the stronger eye, the weaker eye assists rather more the stereoscopic effect of the object under observation. The advantage gained by the binocular is perhaps more appreciated when opaque objects are under examination, as the eggs of insects, and the tongue of the blow-fly, specimens of mosses, lichens, parasites (vegetable and animal), whose planes and inequalities of surface require penetration, and which usually demand more time for their observation. [Illustration: Fig. 45.--Swift-Stephenson’s Erecting Binocular.]