Painters are obliged to avail themselves of this peculiarity, and to make allowances for the double vision. If they do not, the effect of the painting is flat, and it appears as if the artist had only used one eye.
A good proof of this fact may be seen in Stereoscopic photographs, especially of scenery. If each be viewed separately, it often appears quite unintelligible, but, when they are combined by the instrument, they seem to spring into life as it were, and appear solid enough to be grasped.
Now, the Stereoscope is avowedly constructed on the same principle as the double vision of the eye, so that when it applies itself to two photographs of the same object which have been taken from different points of view, it combines them, and gives them as solid an appearance as if they were realities.
So wonderfully close is the representation, that the idea of a place obtained by means of the combination of the photograph and Stereoscope is quite as vivid and correct as if it had been gained by actual observation.
The principle of the Stereoscope is now applied to the best microscopes, and its value is incalculable, especially when low powers are used, i.e. those of not less than half an inch focus. The real beauty of many objects could never have been appreciated but for this discovery, nor their true form defined.
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.