Very interesting and amusing effects are produced by interchanging the right and the left eye pictures in the stereoscope. In general, what was formerly convex is now concave, what was round is hollow, and what was near is distant. The effect of this interchange is finely seen in the symmetrical diagrams, consisting of white lines upon black ground, such as Nos. 1, 5, 9, 12, 18 and 27 of the Parisian set; but when the diagrams are not symmetrical, that is, when the one half is not the reflected image of the other, such as Nos. 26, &c., which are transparent polygonal solids, formed as it were by white threads or wires, no effect, beyond a slight fluttering, is perceived. As the right and left eye pictures are inseparable when on glass or silver plate, the experiments must be made by cutting in two the slides on Bristol board. This, however, is unnecessary when we have the power of uniting the two pictures by the convergency of the optic axes to a nearer point, as we obtain, in this case, the same effect as if we had interchanged the pictures. The following are some of the results obtained in this manner from well-known slides:— In single portraits no effect is produced by the interchange of the right and left eye pictures. If any loose part of the dress is in the foreground it may be carried into the distance, and _vice versa_. In one portrait, the end of the hat-band, which hung down loosely behind the party, was made to hang in front of it. In pictures of streets or valleys, and other objects in which the foreground is connected with the middle-ground, and the middle-ground with the distance, without any break, no effect is produced by the interchange. Sometimes there is a little bulging out of the middle distance, injurious to the monocular effect. In the binocular picture of the Bridge of Handeck, the Chalet in the foreground retires, and the middle distance above it advances. In the picture of the sacristy of Notre Dame, the sacristy retires within the cathedral. In the Maison des Chapiteaux at Pompeii, the picture is completely inverted, the objects in the distance coming into the foreground. In the Daguerreotype of the Crystal Palace, the water in the foreground, with the floating plants, retires and takes an inclined position below a horizontal plane. In the binocular picture of the lower glacier of Rosenlaui, the roof of the ice-cave becomes hollow, and the whole foreground is thrown into a disordered perspective. In Copeland’s Venus, the arm holding the bunch of grapes is curiously bent and thrown behind the head, while the left arm advances before the child. In the picture of the Greek Court in the Crystal Palace, the wall behind the statues and columns advances in front of them.