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 consequence of a single image being sent to his brain. He would, no doubt, after examining the things he saw with his hands, know they were solid, and generally see them so; but if a new object were presented to his view he would have some difficulty in knowing whether it had a flat surface or not. [Illustration: FIG. 67. ]