It will be seen that the eyeball is set in a framework composed of thin bony plates, just like a glass in a telescope. When these plates are relaxed, the whole eyeball is flattened, so as to enable the bird to see an object at a very great distance. But, when they are contracted, they render the whole eye globular in proportion to their pressure, and enable the bird to see objects which are very close to it. In fact, the eye becomes a telescope or microscope as needed. Many reptiles possess this arrangement of bones, but the birds have even a more delicate mode of obtaining the focus of the eye. This is by means of a curious organ called, from its shape, the “pecten,” or comb, which is placed in the vitreous humour at the back of the eye, and connected with the optic nerve. It is a congeries of arteries and veins, so that it can be rapidly enlarged by forcing blood into it, or diminished by allowing the blood to withdraw. 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.