“This singular illusion depends upon the well-known principle, common to optics as to mechanics, that the ‘angle of reflection is equal to the angle of incidence.’ Thus, if a person standing at the point _a_, in Fig. 1, look into a mirror placed in the position indicated by the line _b c_, he will see reflected, not himself, but whatever object may be placed at the point _d_. By an ingenious application of this principle a looking-glass may be used to conceal a given object behind it, while at the same time an image reflected in the glass may be made to represent what would be presumably seen if no glass were there, and thus prevent the presence of the mirror from being suspected. This is the secret of the Sphinx. The table, as already mentioned, has three legs, one in front, and one at each side. Between these legs the spectator sees apparently the curtains at the back of the recess, but really a reflection of the curtains at the sides. The space between the middle leg and that on either side is occupied by pieces of looking-glass (see Fig. 2, which represents a ground plan of the arrangements), extending from _a_ to _b_, and _a_ to _c_. The glass extends quite down to the floor, which is covered with cloth of the same material and color as the surrounding curtains. The {324} spectators, therefore, looking towards the table, see above it the curtains at the back, and below it the reflection of the curtains at the sides; which, however, if the relative angles are properly arranged, appears to be simply the continuation or lower portion of the curtains at the back. The illusion is perfect, and the spectator, from the position assigned to him, cannot possibly discover, by the evidence of his senses, that he is looking at any other than an ordinary bare-legged table, with the background visible in the usual way.