To ensure the full advantage and relief to both eyes in prolonged observations with high as well as low powers, and with objectives of large aperture, Mr. Wenham devised a compound prism for use with his binocular microscope, the body tubes of which are also made expressly to suit the prism, as extreme accuracy is necessary to bring them into proper position. The main prism somewhat resembles in form the ordinary Wenham prism. Over the first reflecting surface is placed a second smaller prism, the top plane of which is parallel with the base of the first, so that direct rays pass through without deviation, but at the two inclined surfaces of the prisms (nearly in contact) there is a partial reflection from each, which, combined, give as much light as in the direct tube. The reflected image from these two surfaces is directed up into the inclined tube as usual. A somewhat later improvement is that of Dr. Schroeder, the high power prism, by means of which the whole of the rays emanating from the objective pass through it, and the full aperture of any power is thereby effectively utilised. Furthermore, Messrs. Ross have also constructed a right- and left-hand pair of eye-pieces, which ensure greater perfection of the image. It was, in fact, noticed that the size of the image in the left-hand field glass slightly differed from that of the right when examined by the ordinary Huyghenian eye-pieces. To compensate for this difference, the left-hand eye-piece has been carefully calculated, and its focus is now so accurately adjusted that the position of each eye in observing is brought into one plane of the binocular. The pairs of the several series of eye-pieces A, B and C have also been altered, and the effect is to greatly improve the image and give increased comfort to the observer. Dr. Carpenter, who warmly espoused the binocular, and constantly employed it in his work, very truly said of it: “The important advantages I find it to possess are in penetrating power, or focal depth, which is in every way superior to that of the monocular microscope, so that an object whose surface presents considerable inequalities is very much more distinctly seen with the former than with the latter.”