candle, he will then see the import of the old proverb, "Two eyes are better than one."
We have said that two eyes are required in order to form a true conception of solidity; this point we now proceed to consider. If the reader will look at any near object, a book placed on end, for instance, he will at once perceive that it is a real book and not a picture of one; he will see that it has a certain relief; that one portion of it is nearer to him than another; in a word, that it is solid. Now, by closing each eye in turn, the reader will find that one eye will see round one side of the object, and the other round the other side, two different impressions being obtained. Every solid object, therefore, is seen differently by the two eyes, and it has been found that the effect of solidity is produced by the combination of these different impressions in the mind. Two eyes are better than one, not merely because they give symmetry to the face, but because they act together in producing on the inner or mental eye, a perfect and instantaneous impression of the form and position of objects.
This important truth has been revealed by the beautiful and well-known instrument called the stereoscope, which, however, is much better known than understood. Some account of this magic instrument certainly merits a place amongst the fairy tales of science.
The stereoscope, in its most popular form, is simply a small wooden box, furnished with two lenses,
