BREWSTER'S STEREOSCOPE.
Sir David Brewster, observing many imperfections in the reflecting type of stereoscope, turned his attention to the subject, and presently brought forward his great improvement, the lenticular stereoscope, which is shown by Fig. 8-12. Brewster conceived the idea that if the principle of refraction was employed instead of reflection, the apparent dis placement of the dissimilar pictures could be effected without loss of light or defini tion, and with the additional improvement of magnification. The principle of this instrument will be understood by Fig. 843.
are being used the workmen will all appear to be left-handed. Lettering on a sign will read backwards, or the right hand side of a street will become the left in the view. Of course, this error may be corrected in the print if desirable, and, in the case of transparencies, they may be placed in a. reversed position in the holders. Apart from these drawbacks of the Stereoscope, there are others, namely, its non-portability, and, worse still, unless special mirrors, silvered and burnished on the face, are used, the pictures will not be seen perfectly defined, there being a double reflection of each picture, one image coining from the metallic surface and a fainter image from the surface of the glass. With all its
defects, however, this instrument marked a notable advance in viewing devices.
Two convex prisms, G and a, are fitted into the framework, the two pictures to be examined being situated at A a and o D. The function of the convex prisms G and ti is to refract the light emanating from the actual pictures, so that the eyes situated at L and R see their respec tive pictures at E r combined, magnified, and giving together stereoscopic effect. The lenticular stereoscope has been made in numerous forms, and its box form with partition and reflector (Fig. 812) has long been superseded by more portable ap pliances. The American form of lenticu lar stereoscope (Fig. 844) was designed by Oliver Wendell Holmes. The view carrier D can he moved backward or forward for focussing purposes. The hood at the front serves to exclude superfluous light, the inlet A being a recent addition to aid the camera obscura effect. For con venience in packing, the handle B is hinged to the underside, and may be folded backwards to the dotted position c, while the holder D may be taken off.