A Manual Of Photography

THE STEREOSCOPE. 305 an angle of ninety degrees with each other. These mirrors are fixed by their common edge upon an horizontal board, in 84. such a manner that, upon bringing it close to the face, each eye sees the image reflected from the two ends of the instrument in a different mirror ; at each end of the board are panels in whicli the drawings are placed. The two reflected images coincide at the intersection of the optic axes, and form an image of the same apparent magnitude as each of the component pictures. This instrument is called the reflecting stereoscope ; and as it will admit of being made of any size, so as to allow of the intro- duction of large pictures, it offers many advantages. Mr. Wheatstone suggested in his memoir, already quoted, the use of an instrument constructed with prisms, whicli is analogous to the beautifully portable lenticular stereoscope of Sir David Brewster, described by him in the Philosophical Magazine. This instrument consists of two semi-lenses, placed at such a distance that each eye views the picture or drawing opposite to it, through the margin of the semi-lens, or through parts of it equidistant from the margin. A lens, a a, being cut in two 85. halves, these are fixed into a frame L and R, and adjusted to sucli distances that the centres of the semi-lenses correspond with the pupil of the eyes. The distance of the centre of one pupil from the other is at an average 2| inches, and to this the semi-lenses may be adjusted; but if the instrument is provided with the means of effecting a little change in this respect, it will often be found to be of considerable advantage. X