As the liquid in which it rests is practically incompressible, it follows that when the comb expands, it causes the chamber of the vitreous fluid to expand, and so forces the lens forward. When, however, the blood retires from the comb, the lens returns to its original place. This, as the reader may have noticed, is the same principle as that which is followed in altering the focus of a telescope in order to suit the sight of different individuals. Perhaps a still better illustration may be found in the coarse and fine adjustment of the microscope, the former of which moves the whole tube, and may be compared to the bony ring; while the latter causes one part to slide over the other, and is analogous to the comb.
The movements of this organ are believed to be as involuntary as the dilatation and contraction of the iris; but, whatever may be the case, it is one of the most beautiful examples of natural mechanics, and far surpasses the most delicate machine that can be made by man.
In the illustration of the microscope, which is to be found on [page 286], both these movements are given, the double vertical wheel being the coarse movement, and the fine movement being supplied by the single vertical wheel just above them.
While we are on this subject, we may see how Art unintentionally copies Nature, even in trivial details. Every one who is in the habit of using optical instruments, more especially those who are forced to wear spectacles, are aware of the necessity of keeping the glasses as clean as possible, and, where the instruments are delicate, always have by them a piece of clean wash-leather for the express purpose of wiping the glasses.
Here, again, Nature has anticipated Art. In our own case, we have in the human eye a good example of such natural mechanism, the eyelids being formed quite as much for the purpose of washing the surface of the eyeball as of excluding light.
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.
