In Mr. Wheatstone’s stereoscope he employs two mirrors, each _four inches_ square—that is, he employs _thirty-two_ square inches of reflecting surface, and is therefore under the necessity of employing glass mirrors, and making a clumsy, unmanageable, and unscientific instrument, with all the imperfections which we have pointed out in a preceding chapter. It is not easy to understand why mirrors of such a size should have been adopted. The reason of their being made of common looking-glass is, that metallic or prismatic reflectors of such a size would have been extremely expensive. [Illustration: FIG. 28.] It is obvious, however, from the slightest consideration, that reflectors of such a size are wholly unnecessary, and that _one square_ inch of reflecting surface, in place of _thirty-two_, is quite sufficient for uniting the binocular pictures. We can, therefore, at a price as low as that of the 4-inch glass reflectors, use mirrors of speculum metal, steel, or even silver, or rectangular glass prisms, in which the images are obtained by total reflexion. In this way the stereoscope becomes a real optical instrument, in which the reflexion is made from surfaces single and perfectly flat, as in the second reflexion of the Newtonian telescope and the microscope of Amici, in which pieces of looking-glass were never used. By thus diminishing the reflectors, we obtain a portable tubular instrument occupying nearly as little room as the lenticular stereoscope, as will be seen from Fig. 28, where ABCD is a tube whose diameter is equal to the largest size of one of the binocular pictures which we propose to use, the left-eye picture being placed at CD, and the right-eye one at AB. If they are transparent, they will be illuminated through paper or ground glass, and if opaque, through openings in the tube. The image of AB, reflected to the left eye L from the small mirror _mn_, and that of CD to the right eye R from the mirror _op_, will be united exactly as in Mr. Wheatstone’s instrument already described. The distance of the two ends, _n_, _p_, of the mirrors should be a little greater than the smallest distance between the two eyes. If we wish to magnify the picture, we may use two lenses, or substitute for the reflectors a totally reflecting glass prism, in which one or two of its surfaces are made convex.[40] [40] We may use also the lens prism, which I proposed many years ago in the _Edinburgh Philosophical Journal_. 2. _The Single Reflecting Stereoscope._