THE STEREOSCOPE.
Having devoted so much space in the preceding
chapters to optical amusements of a purely recreative
character, it is only right that we should now say a few
words on certain instruments of a less frivolous character
than those we have lately been considering, and which
deserve at our hands the most serious attention. We
shall, therefore, in the present chapter, speak of an ingenious
instrument which serves to show in relief the
images of objects depicted on a flat surface. We have
already seen, that although we have two eyes, provided
with lenses and screens by means of which the images
of things around us are formed, we only perceive
a single object; and the student has no doubt long
since wondered why nature has bestowed two eyes upon
us, when only one would have apparently served the
same purpose. This question was for a long time a
complete puzzle to philosophers, and it was not until
Professor Wheatstone made his experiments on binocular
vision in 1838, that the matter received a satisfactory
explanation. He showed that each eye receives a different
impression of any object upon the retina, and that
it is in consequence of the union of these slightly dissimilar
images that the sensation of relief is experienced.
A one-eyed man or a Cyclops would only partially
perceive relief in the objects presented to his view, in
