FIG. 150.—Common form of prism stereoscope.] =Stereoscopes.=—The easier and more usual method of fusing the stereoscopic images is by a _stereoscope_. The simplest form consists merely of two convex lenses, one for each eye, their centers separated by a distance somewhat greater than that between the eyes. Their function is to bring the stereogram to focus, and, by the prismatic action of the edges of the lenses, to converge the lines of sight which pass through the centers of the two pictures to a point in space in front of the observer. The two lenses should be mounted so as to provide for the adjustment of their separation to fit different eyes and print spacings. The most common form of stereoscope is that designed by Holmes, for viewing paper print stereograms (Fig. 150). It has prismatic lenses of an appropriate angle to converge pictures whose centers are three inches apart, instead of the lesser distance appropriate to stereograms intended for fusing without an instrument. No adjustment is provided for varying the lens separation, but the print can be moved to and fro for focussing. [Illustration: FIG. 151.—Box stereoscope.] [Illustration: FIG. 152.—Diagram of mirror stereoscope.] Another form of stereoscope, one of the first produced, is the mirror stereoscope (Fig. 152), now used extensively for viewing stereo X-ray pictures. It consists of two vertical mirrors at right angles to each other, with their edge of contact between the eyes. The two prints to be studied are placed to right and left, an arrangement that permits the use of prints of any size. The convergence point is controlled by the angle between the mirrors. The Pellin stereoscope (Fig. 153) utilizes two pairs of mirrors in a way to permit the use of large prints. The prints are, however, placed side by side on a horizontal viewing table, which avoids certain difficulties of illumination met with in the simpler mirror form. The box form of stereoscope (Fig. 151) using either prisms or simple convex lenses, is particularly adapted for viewing transparencies, although the insertion of a door at the top provides illumination for paper prints. The Schweissguth design (Fig. 154) is intended primarily as an aid to selecting the portions of the prints to be cut out for mounting. The platform on which the pictures rest is composed of two long rectangular blocks, on which are plates of glass raised sufficiently to permit the prints to be slid underneath. The space between the blocks allows the unused portion of the photograph to be turned down out of the way. Prints of any size can thus be moved about until the proper portions for stereo mounting are found. Either block can be moved in its own plane and also to and from the eye, whereby two prints of somewhat different scales can be fused. [Illustration: FIG. 153.—Pellin double mirror stereoscope.]