BINOCULAR MICROSCOPE, a microscope adapted to be used by both eyes at the same time. It has only one set of object glasses, but the pencil of light, after pass ing these lenses, is divided, and the parts are sent to the eyes separately. The division is caused by a trapezoidal prism that is pushed lattcrally into the pencil of light, cutting off one half; the other half goes on directly to one eye. That part of the pencil which is obstructed enters the lower face of the prism normally and is not there changed; it meets the second face internally at such an angle as causes it to be wholly reflected and to pass back through the glass to the third face; here it is again totally reflected. and it passes thence out of the glass normally through the fourth face. The result at all these changes of direction is to give it a path, slightly oblique, to that of the unchanged ray, that will carry it through an oblique tube to the second eye. The rays of light cross in
the objective; hence, to obtain a stereoscopic effect—that is, to cause the object to stand forth as a solid, its three dimensions being properly appreciated—the light which comes from the left side of the object must enter the right eye, and vice versa. Should the light from the right side enter the right eye, a pseudoscopic effect follows; projections seem hollows, and hollows look like elevations. The binocular microscope has two eye pieces. It is restful to the eyes, and with ]ow powers gives information not to be had otherwise, showing the depth, as well as the length and breadth, of the thing observed. The binocular- telescope has two tubes and two sets of lenses throughout. A pair of opera-glasses is a familiar example.