The former is constructed by placing two upright boards on a wooden stand at a moderate distance from each other; the stereoscopic pictures are attached to these boards, which may be made to move up or down, and if the pictures are held in grooves, they may be pulled right or left at pleasure, and thus four movements are secured--viz., upward, downward, right, or left. Between the two stereoscopic pictures are placed two looking-glasses, so adjusted that their backs form an angle of ninety degrees with each other. (Fig. 308.) [Illustration: Fig. 308. Wheatstone's reflecting stereoscope.] The pictures are illuminated at night by a lamp or gas flame placed at the back of the mirrors, which, when fixed together, have the same shape as a prism; indeed, Professor Wheatstone substituted a prism for the mirrors, and thus paved the way for the invention of the lenticular stereoscope. [Page 322] The stereoscopic effect is obtained by bringing the eyes close to the inclined mirrors, so that the two reflected images coincide at the intersection of the optic axis; the coincidence of the images is further secured by moving either picture a little to the right or left, and if the upright boards move bodily in grooves to or from the centre mirror, the greatest nicety of adjustment is procured. During the last three years of the author's directorship of the Polytechnic--viz., in 1856, 1857, 1858--nearly the whole of the pictures shown by the dissolving-view apparatus were coloured photographs from Mr. Hine's original pictures, painted two feet square in blue and white, and reduced on the glass to about six inches square. The collodion film being frequently thick and difficult to penetrate with light, was etched and scratched away where required, and filled in with colour, and when these pictures were looked at with _one_ eye only, they appeared to be almost solid or stereoscopic on the disc. The lenticular stereoscope consists of a box of a pyramidal shape, open at the base, and provided with grooves in which are placed the stereoscopic pictures; if the latter are taken on glass the base of the box is held directly against the light, but if they are daguerreotypes or paper pictures, then a side light is reflected upon them by means of a lid covered in the inside with tinfoil, which is raised or lowered at pleasure from the top part of the box. Two semi-lenses are now fitted into the narrow part of the box, and are placed at such a distance from each other that the centres of the semi-lenses correspond with the pupil of the eyes, and this distance has already been stated to amount to 2½ inches. (Fig. 309.) [Illustration: Fig. 309. Brewster's lenticular stereoscope.]