Many animals are provided with a special apparatus for the purpose, called the “nictitating membrane.” It is, in fact, a sort of inner or supplementary eyelid, which can be drawn over the eye while the external lids remain comparatively unmoved. It is very conspicuous in the owls, and gives to those birds that almost comical look of perpetual blinking with which we are so familiar. THE STEREOSCOPE AND PSEUDOSCOPE. Many persons have wondered how it happens that, as we have two eyes, we do not see two images instead of one. Practically, this is always the case, for the eyes, especially when they look on solid bodies, see two different images, because they contemplate the object from different points of sight. This may be easily ascertained by looking at a given object first with one eye, and then with the other, when it will be seen that the image presented to the right eye is slightly different from that of the left eye, but that the two can be combined into one by a very slight inward movement of both eyes, and thus the effect of a solid body be produced. Sometimes, when people are weak, and cannot control the united movement of the eyes, not only two, but five or six images are at once presented to the mind, and produce a strange sense of bewilderment and confusion. [Illustration: COMBINATION OF HUMAN EYES.] [Illustration: STEREOSCOPE.] [Illustration: BINOCULAR MICROSCOPE.] 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.