The Camera Obscura and the Eye.
I have already spoken of arts as being akin to each other. They are more than this, and every day of the world’s progress teaches us that Art, Science, and Manufacture are sisters, all born of one family, and all depending mutually on each other.
Take, for example, our present theme—namely, Optics—and see how dependent it is upon Manufacture and Art. Without the former, man could not construct those beautiful telescopes, microscopes, spectroscopes, of the present day, which are evidently but the precursors of instruments which will work still greater marvels.
The first enables us to see solar systems without number, to which our own, vast as it seems to us, is but as a grain of sand in the desert. The next instrument makes revelations as marvellous of the infinitely minute as does the telescope of the infinitely great, enabling us to see living organizations so small that thirty-two millions could swim in a cubic inch of water. The third, a comparatively modern instrument, reveals the composition of objects, and can detect and register the materials of which the sun and fixed stars are made, or detect an adulteration in wine. It can adapt itself equally to the telescope and microscope, and the very same instrument which will reveal the character of an invisible gas in the Pole-star, when attached to the telescope, can, when connected with the microscope, point out the presence of half a corpuscle of blood where no other instrument could discover any trace of it.
All these instruments, together with many others, will be described in the present division of the work, and their analogies with Nature shown.
We will now take the subject of the Camera Obscura, an instrument with which the photographic apparatus of the present day has made most of us familiar. As its action depends chiefly upon the glass, or lens, through which the rays of light pass into the instrument, we will first explain that.
A “lens” is a glass formed in such a manner that the rays of light which pass through it either converge to a focus, or are dispersed, by means of the law of refraction. Every one who has been photographed—and who has not?—will remember that when the sitter has taken his position, the photographer brings to bear upon him a circular glass fixed into a short tube, and then looks through the instrument as if he were taking aim with some species of firearm. It is no matter of wonder that when savages see the photographic camera for the first time they are horribly frightened, for there is really something weird-like in the appearance of the lens thus presented.
Now, this lens is of the shape called “double convex,” both sides being equally rounded, so that a section of it would be shaped very much like a parenthesis (). The effect of this form of lens is to bring the rays of light to a point at a given distance from the centre. This point is called the “focus,” and is well known by means of the common burning-glass, which will set fire to objects placed in its focus, while itself remains quite cool.
I have seen lead pour down like water when placed in the focus of a large burning-glass, and even the harder metals will yield to the power of the sun’s rays when thus concentrated.
There is nothing which gives a more vivid idea of the amount of heat thrown on the earth by the rays of the sun than the effects of a moderately large burning-glass—say one of six inches in diameter. If we trace a circle of this size on the surface of the earth, it does not seem as if any very great amount of heat can be received, but when we catch the rays of that circle in our glass, and bring them together upon the focus, the amount of heat can be appreciated. The well-known meridian gun in the Palais Royal is fired by the sun. A burning-glass of no very great size is placed over the touch-hole of the gun, with which its focus coincides. The lens is turned in such a manner that, as the sun attains the meridian, its rays are thrown upon the touch-hole, and consequently fire the gun.
The word focus is the Latin term for a domestic hearth, and is used in allusion to the heat which is manifested at the point on which the rays of the sun converge.
It is evident that, after reaching the focus, the rays, if they be not intercepted by some object, will cross each other, and form a large image, but reversed. This part of the subject will presently be explained.
The accompanying illustration shows two figures, one representing the section of a double convex lens made by the hands of man, and the other that of a double convex lens as seen in Nature.
The former has already been explained. The latter is the double convex lens of the human eye, by means of which the images of external objects are conveyed to the brain. Whenever this lens becomes thickened by disease, the sight is gradually dimmed, and at last total blindness is the result. This disease is popularly called “cataract,” and until late days was incurable. Now, however, any good oculist will attack a cataract, and either partially or entirely restore the sight. This operation is performed by carefully removing the convex lens, and supplying its place with a glass lens, which throws the rays of light on the same focus.
The figure shows the double convex lens of the human eye in its place.
Having now seen something of the properties of the double convex lens, we will examine its application to the Camera Obscura.
The lens is placed on one side of the camera, and is so made that it can be slid backwards and forwards, and the focus altered at will. The camera itself is a box completely closed, so that no light can enter it except that which passes through the lens. The latter is so arranged that the rays which pass through it are crossed, and throw their image on the opposite side of the camera. In the photographic camera a piece of ground glass is placed at the end, so that the rays fall upon it, and the operator can see whether the image is a good one. Of course the figures are reversed, so that the sitter seems to be on his head, but that is a matter of no consequence. Exactly the same effect is produced by the marine telescope.
The general structure of the camera is shown in the illustration, all needless details being omitted.
I may here remark that the term “camera obscura,” or dark chamber, alludes to the fact that the box is completely closed, and, but for the rays which pass through the lens, would be absolutely dark.
The opposite illustration shows the most perfect camera obscura that can be imagined, namely, the human eye. Here we have a dark chamber, a double convex lens, and an image falling upon the back. Here the optic nerve comes into play, takes cognisance of the image, and conveys the idea to the brain. With a little trouble, a real eye, say that of an ox, can be dissected out, and employed as a camera obscura, the operator seeing in the back of the eye, or “retina,” the same image which the ox would have seen if it had been alive.
In photography, the operator, when he has found that a perfect image is thrown upon the ground glass, which represents the retina of the eye, substitutes for it a sensitive surface, on which the rays are projected, and which, by chemical means, produce a permanent instead of a fleeting object.
Examples of other lenses may be found in Nature. She, moreover, can perform a task which man has never even attempted, namely, the change of form in a lens according to the duty which it has to do. How this wonderful object is attained we shall presently see.
There is a form of lens extremely useful in Optics, namely, the “Plano-convex” lens. This is, in fact, one half of a double convex lens, the section being made through its edges, and the plane sides polished as well as the convex. As, however, this is only a half of the double convex lens, it does not need further explanation. Its natural counterpart may be seen in the annexed illustration.
A somewhat more complicated form of lens is called the “Meniscus,” one side of which is convex and the other concave. A good example of the meniscus may be found in the old-fashioned watch-glass, before watchmakers took to flattening them, and watch-wearers were not ashamed to carry a “turnip,” in which there was room to spare for the works. If a section of such a glass were taken, it would assume the form of a half-moon. This, in fact, is the meaning of the term “meniscus,” which is a Greek word, signifying a little moon. If the same glass were solid, or even filled with water, it would form a “plano-convex” lens.
Of course the outer curve of the meniscus must be larger than the inner curve, but in some cases the disproportion is very strongly marked, the outer curve being very large, and the inner curve very small. An example of such a meniscus may be seen in the human eye. If the reader will refer to the illustration on [page 280], in which the structure of the eye is shown, he will see the meniscus lens in combination with the double convex. The former has already been explained, and the latter is formed by the vitreous humour which fills nearly the entire globe of the eye. Its larger curve is due to the form of the eyeball, and the smaller to the convex lens.