like an opera-glass. A double picture, say a photograph of a statue, is placed at the bottom of the box, and viewed with both eyes, by means of the lenses. The effect is truly marvellous, for the design immediately appears in relief the picture becomes a piece of sculpture! This illusion is so perfect, and the means by which it is produced so simple, that we cannot wonder at the popularity which the stereoscope has so rapidly attained.
The term stereoscope is derived from two words in the Greek language, the first signifying a solid body, and the latter vision; it may therefore be freely translated as "that which shows every object in relief." Our readers will admit that the name is a good one, and perfectly descriptive of the powers of the instrument.
Let us now consider how the wonderful illusions of the stereoscope are effected. We shall not require diagrams to make our meaning clear, since every one must be familiar with the construction of the magic instrument.
The two pieces of glass that are placed in the front of the stereoscope are wedge-shaped, that is to say, their outer edges are a little thicker than their inner edges. These glasses act like prisms, and by bending the rays of light that proceed from the double picture, they cause the two halves to combine, and appear as a single picture occupying a central position between the eyes. Two distinct images are thus formed in the eyes, but in conse-
