On the left hand of the illustration is shown the combining power of the eyes. Supposing the right eye only to be brought to bear upon the little cylinder, only one side of it will be seen, and it looks nearly flat. The same is the case with the left eye. But, when both eyes are used together, both sides of the cylinder are presented to the mind, and thus we get the effect of solidity. The Stereoscope is so formed, by means of lenses, that the two figures become combined into one, the rays of light being turned out of their course by the arrangement of the glasses. The Stereoscope, however, although a useful assistant to the vision, is not necessary. It is perfectly possible to combine the two figures without any stereoscope, and to do so merely by squinting, if we may so call it, at the figures. The power of combination is gained with a very little practice, and in a short time the observer will be capable of producing stereoscopic effects without needing a Stereoscope. This ability is very useful when inspecting photographs in a shop-window. Of course the figures are not so much enlarged as they are with the stereoscope, but they are nevertheless quite as clear and well defined. * * * * * THERE is an instrument called the Pseudoscope, which, as its name imports, gives a false idea as to the nature of the object which is viewed through it, converting hollow objects into solid, and _vice versâ_. The following description of its effect is given by Wheatstone:-- “When an observer looks with the pseudoscope at the interior of a cup or basin, he not unfrequently sees it at first in its real form; but by prolonging his gaze he will perceive the conversion within a few minutes; and it is curious that, while this seems to take place quite suddenly with some individuals, as if the basin were flexible, and were suddenly turned inside out, it occurs more gradually with others, the concavity slowly giving way to flatness, and the flatness progressively rising into convexity. “Not unfrequently, after the conversion has taken place, the natural aspect of the object continues to intrude itself, sometimes suddenly, sometimes gradually, and for a longer or shorter interval, when the converse will again succeed it--as if the new visual impression could not at once counteract the previous results of recent experience. At last, however, the mind seems to accept the conversion without further hesitation; and after this process has once been completely gone through, the observer, on recurring to the same object, will not find it possible to see it in any other than its converted form, unless the interval should be long enough to have allowed him to forget its aspect. “Vagaries, however, sometimes occur in these experiments of which it is difficult to give any certain explanation, but which would be probably found referable to the same general principle, if we were acquainted with all the conditions of its operation.”