Page:The fairy tales of science.djvu/139

Rh discovery of the stereoscope, a discovery which Herschel has truly characterized as "one of the most curious and beautiful for its simplicity in the entire range of experimental optics." The original form of the instrument has been considerably modified by Sir David Brewster, who may indeed be regarded as the inventor of the refracting or popular stereoscope.

We will not attempt to describe the innumerable family groups, landscapes, portraits, and Alpine views, that photography has furnished for the stereoscope. Our readers are doubtless familiar with them, as the stereoscope has become quite a fashionable instrument, and has taken the place of the album upon almost every drawing-room table.

Two eyes are unquestionably better than one, nevertheless persons with but one eye are able to see distinctly. This fact does not refute what we have said about double vision. A person with one eye judges of the relief of an object from the distribution of light and shade, but his perceptions are much less vivid than those of a person with two eyes. It has, moreover, been remarked that a one-eyed person when looking at a solid object is constantly changing the position of the head from side to side, and by this means he obtains with one eye the same result that is obtained by two eyes with the head stationary.

Our readers will now understand why they have two eyes instead of one, and will be able to expound the mysteries of that magic spy-glass, the stereoscope.