Long and Short Sight.
It has already been mentioned that the focus of a convex lens is shorter in proportion to its convexity, and that in consequence its magnifying power is increased. For example, the large glasses through which pictures are viewed are comparatively thin in proportion to their diameter, while the lenses employed for the highest powers of the microscope are scarcely larger than small shot, and nearly as globular. It naturally follows that any instrument to which a lens is adapted, whether it be microscope or telescope, must depend for its focus on the greater or less convexity of the lens in question.
Again taking as our example the human eye, we find that there are very few persons who from youth to age possess or preserve eyes which can read small type at a moderate distance, and can clearly define the outlines of distant objects. Nearly all people, even if in their youth they possess good sight, lose it as they grow older. They can discern distant objects well enough, but, when they come to reading, they are obliged to hold the book at arm’s length before they can distinguish the letters.
This defect is caused by the insufficient convexity of the lens, so that the focus is thrown too far back, and it is corrected by wearing spectacles sufficiently convex to supply the deficiency in the lens of the eye.
An admirable example of temporary long-sightedness is familiar to every diver, though he may be unconscious of its cause. Suppose that into very clear water of some twelve feet in depth, a white object, say a common jam-pot, is thrown, it can be clearly discerned from the shore, unaltered in shape or size. But, when the diver searches for it, he sees at first only something white, large, undefined, and wavering, and only finds it resume its proportions as he approaches it. This phenomenon is due to the pressure of the water upon the eyeball, which flattens it, and so throws the focus too far back for a clear image. Nowadays this defect is remedied by the use of very convex spectacles, so convex, indeed, that, if worn in the air, they would render the wearer incapable of seeing anything at more than an inch or so away from him. But, when worn in the water, they only supply the deficiency of the compressed eyeball, and so restore the focus to its proper position.
Those who suffer from short-sightedness can see with great distinctness objects which are close at hand, but those at a little distance seem to have no particular outline, and appear as if they were viewed through a fog, thus causing a constant and almost painful strain on the eyes. The cause of this defect is the too great convexity of the lens, which therefore throws its focus short of the required spot. The means of remedy are exactly opposite to those which are used for long-sighted persons, a concave lens being placed in front of the eye, so as to throw the focus farther back, and relieve the organ from the strain.
Although we have not yet invented a machine that can alter the focus at will, we may take a hint from Nature. We have already seen how the pressure of water upon the front of the eye lessens its convexity, and makes it long-sighted. Consequently, if we could apply pressure round it, we could make it more convex, and so neutralise the weight of the water.
There is a wonderful piece of machinery in Nature which really does perform this office, the eye, at the will of its owner, becoming either telescopic or microscopic. This quality is very desirable in birds, especially those which are predacious and of rapid flight, as they might either fail to see their prey at a distance, or might dash themselves against some obstacle when they were close upon it.
The eye of the Owl affords a beautiful example of machinery which produces this effect, and the means which are used may be understood by inspecting the accompanying illustration.
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.
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.