A combination of the principles of the phenakistiscope (fig. 4) and stereoscope, has resulted in the invention of an instrument called the stereotrope. A number of binocular photographs of some object in motion—a steam-engine, for instance—are taken when the moving parts are in different positions, and mounted on two revolving discs, the images being combined by means of a pair of semi-lenses, as in the ordinary refracting stereoscope. We cannot leave this subject without describing the pseudoscope, also the invention of Professor Wheatstone. If a stereoscopic pair of photographs of some solid body—a ball, for instance—are mounted the reverse way, that is to say, if the picture intended to be looked at by the right eye is placed on the left, the relief of the object will be reversed, and the ball will appear as a hollow hemisphere. If, therefore, we can by means of lenses or prisms cause the image of any natural object, as seen by the right eye, to be conveyed to the left, and _vice versâ_, we shall see the relief reversed. A conical cap will appear in relief as a cone, a globe will look like a hollow sphere, and the human face will take the semblance of the inside of a mask. The same deception may be effected by looking at a seal through a short-focused lens, so that the image shall seem reversed. In this case, the light coming apparently from the wrong side, and shining on the parts in relief, gives them the appearance of being hollow. An intaglio will, of course, appear in relief when so looked at. Photographs of gems and bas-reliefs will also present a pseudoscopic appearance, if looked at in a light coming from the opposite side to that in which they were taken. The same appearance may be seen sometimes in wall papers having patterns painted in strong relief. ------------------------------------------------------------------------ CHAPTER VIII. THE CAMERA OBSCURA AND CAMERA LUCIDA.