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VA A T has been eloquently said that " while the micro
scope unfolds to us a world of life and enjoyment in n every atom, the telescope enables us to see a system
in every star, and suggests that, above and beyond all that is visible to man, there may be regions of creation which sweep immeasurably along, and carry the impress of the Almighty's hand to the remotest scenes of the universe."
The one reveals to us the wonders of the organic world, displaying beauty and harmony of structure in beings otherwise but dimly visible, and detecting activity of being where life was altogether unsuspected by the unassisted eye; the other makes known to us the superficial character of orbs, which seem mere luminous specks to the natural vision, and draws forth from the recesses of space stars and world-systems, the existence of which, but for its aid, would remain for ever unknown. The one exhibits the immensity of the Creator in the minute; the other his minuteness in the immense. Like all human means, however, both are limited and imperfect, and leave the enlightened imagination to depict myriads of forms and functions in either direction as perfect, and as perfectly sustained, as those which are the nearest and most familiar. To the revelations of the microscope, we have already devoted a number of this Miscellany; the still more marvellous discoveries of the telescope will form the subject of the present paper. The objects of the former lie amid the minute of our own world ; those of the latter are the countless suns and sun-systems scattered throughout the regions of illimitable space. It is necessary, however,
before detailing results, to explain in some measure the nature of the instrument by which these results have from time to time been determined.
The word “telescope," like most of our scientific terms, is derived from the Greek, and signifies the seeing of things at a distance. The natural eye is formed to see objects within a certain range, but by the help of the telescope, we can discern objects that are too distant to be seen by the naked eye; and the things that the eye of itself can see only in a dim and indistinct manner, may be seen by that instrument as clearly as if close at hand. The invention of this wonderful aid has been ascribed to various persons; but whoever the inventor, it is certain that it was not directed, in the spirit of philosophical inquiry, to the heavenly bodies anterior to the beginning of the seventeenth century (1609-10), when the illustrious Galileo, and our countryman Harriot, simultaneously, but unknown to each other, commenced that career of observation which, with all its imperfect appliances, has already disclosed so many marvellous and important facts and existences.
The powers of the telescope depend upon the action of glass lenses upon rays of light. À lens is a piece of glass (such as the eye of a pair of spectacles) whose surface is not flat like window or mirror glass, but rounded, so as to be either thicker in the middle than at the edges, which makes a convex lens, or thinner in the middle than at the edges, making a concave lens. In fig. 1, page 5, the lenses A B and C D are both convex; in fig. 2, A B is convex, and C D concave.
When light passes through a piece of plane glass, such as a window-pane, the rays proceed in the same direction after passing through as they had before entering. A level ray would continue level on coming through a piece of plane glass; but if, instead of plane glass, we use a lens, the direction of the light is altered in going through. A level ray may be turned up or turned down, according to the surface-line of the glass; and a north ray would diverge to the east or west. This bending of the rays alters very much the appearance of the bodies they come from. If we see a man through a window, we see him exactly as if there were nothing between him and us; but if we see him through a lens, his appearance is totally altered, owing to this bending of the rays from his person in their passage through the lens. If it be a concave lens, the bending will be such, that he will seem smaller and farther off than he is. The rays will strike the eye in the same way that rays would strike it from a man standing at a greater distance. If it be a bulging or convex lens, he will seem larger and nearer than he actually is, when
we hold the lens at a certain distance from the eye. If we hold it at other distances, his whole image will be broken up, and made so incoherent, that we will not recognise even the shape of a human being. This arises from the directions of all the rays being so much altered, that they do not enter the eye in the proper manner for producing the figure of a man; but there is one position, at a particular distance from our eye, that will show the image of the person we are looking at correct and complete, but turned upside down, and made perhaps much smaller or much larger than the reality. This is the most singular effect of all. The rays from all parts of his person pass through the lens, and are so bent inward by it, that they all cross one another on the other side, and fall into their proper places, so as to make a new picture, as it were, in the air, whose rays proceed to the eye, and show a new inverted man, instead of the original upright one. If a lens be correctly rounded, and well placed, it has this remarkable power of creating a new image of all objects that are exposed to it: the object being on one side of the lens, and the image, as seen by the eye, on the other. All the rays coming from the head of a man are so bent in passing through the lens, that they join together again, and make a new image of a head; whilst those from the breast fall together next the head, and so on; every part, and every ray, being nearly in the same place in the new picture as in the original, only the whole is turned upside down. Any one may make this interesting experiment with a magnifying glass. If the glass is properly held before a candle, a new inverted candle will be pictured in the air on the other side ; and the picture may be seen by the eye, if it looks in the straight line of the glass and the candle, or the picture may be brought out on a sheet of paper held on the other side of the glass from the candle, and moved backward and forward till the place is found where the image is most correct. The place where the image is formed is called the focus of the lens. The camera obscura and magic lantern are constructed with a single convex lens on this principle. They enable us to transfer the whole appearance of any object to a piece of paper, or a screen, or a bit of frosted glass; and, by the choice and placing of the lens, we may make the new picture either larger or smaller than the original.
This wonderful power of a single bit of rounded bulging glass, to create between the eye and the glass a new representation of any outward thing, may be turned to the most valuable purposes. It may be so managed, that a very distant object can be viewed with as great distinctness as if it were many times nearer. If we can so arrange matters, that our eye shall come very close to this new image, we may be able to see its parts with very great minuteness; for in this case we are not dealing with the original, which may be many yards, miles, or even millions of miles off; we are, in fact, viewing a thing within a few inches
- perhaps of the eye. But it is necessary to explain how we can take a close view of such a nice object as an aërial picture.
If we use a screen, as already explained, the thing will be before us as it were bodily, and we can examine it with our eyes; or we may use a magnifying glass or a microscope to make it still more visible. But the same thing can be done without a screen. It is, however, necessary to understand precisely the use of a single lens as a magnifying glass, which is a convex lens the same as the picturing glass, only it is held close to the eye, instead of being placed at a considerable distance. When held close, it does not make a new picture-it only modifies the appearance of the original, so as to make it larger, as is generally supposed; but, in fact, this is not what really happens. The real action of a magnifying eyeglass is this: there is a certain distance from the eye where things close at hand are very distinctly seen ; if they are brought nearer, they become confused and indistinct. Thus an ordinary person sees small print well at about six or eight inches from his eye; if he bring it within four inches, it becomes unreadable. The eyeball itself is a convex lens, acting, as we have explained, by making on a screen at its back surface an image of all outward objects; and this image is not perfect if things are held too near. But if we are looking at a thing that is very small, or very nice and minute in its parts—such as the parts of a small insect-we hold it very near, that it may cover a larger space on the eye, and make a larger picture for us in the inside. Indeed, in looking at very fine objects, we would hold them to the very surface of the eyeball itself, to gain more apparent size, if it were not for the confusion already mentioned. Now, it is the office of the magnifying glass to remedy this confusion, and allow the things to be placed close. It so modifies the direction of the rays of light, that an object that made a distinct appearance only at eight inches from the eye, may be distinct at four inches, or even one inch-perhaps a quarter of an inch, if the form be very much rounded, like a little glass bead. Every one may observe that in using such a glass, he holds the object looked at closer to the eye. But if an object can be distinctly seen at four inches from the eye, it makes four times as large a figure as when it stands at eight inches, and its small parts will thus be fourfold more apparent: a point scarcely noticed in the latter case, may be very distinct if it be doubled in length and doubled in breadth, as it really is at half the distance from the eye. If, therefore, distinct vision can be made to take place at one inch, the object will really be eight times larger each way than at eight inches; the surface will have sixty-four times the extent, which must enable far smaller parts to be seen. Such is the magnifying function of a convex lens-its power of modifying the appearance of an outward object by diverting or bending the direction of its rays.
Now, a telescope contains two lenses : one is used to create a
picture; the second to magnify that picture, or, more properly, to enable the eye to come very close to it, and yet have a distinct picture of it. Two convex
Fig. 1. lenses properly adjusted, constitute the simplest form of the telescope, as in fig. 1.
Let the arrow (MN) stand for any outward object, and let its rays fall upon a convex lens (AB), which is seen edgeways in tbe figure. The rays will be all bent, so that, at a certain distance on the other side of the lens, a new representation of the arrow (nm) will be made. The rays from the point of the arrow at M will be so acted on at the two surfaces of the lens, that they will all come together again, and make, as it were, an arrow point at m. The rays from the feather at N will fall into their places at n, in a new arrow head; and so on throughout, the whole being inverted. A second lens (CD) is used, not to form a second picture (as it would do if distance were allowed it), but to enable the eye at E to look closer at nm than it could otherwise do. What the eye sees, therefore, by the two lenses, is a near picture of the original arrow turned upside down. This picture is nearer and larger to the sight in proportion to the roundness of the magnifying lens. Suppose the image (nm) is 6 inches from the picturing or object lens, then, if the eye look at it at a distance of 6 inches, the picture will have the same apparent size as the original, and nothing will be gained. But if the second lens, called the eye-piece, enables the eye to come within 1 inch of the picture, and yet see it without confusion, it will be 36 times as large to appearance as the original, 6 times each way. In fact, the view is now improved as much as if a six-mile object were brought within 1 mile. Now, the greater the distance of the picture from the object lens which forms it, the greater its focal distance; and the nearer that the eye can be brought to the picture by the eye-lens, the larger the appearance will be, or the greater will be the magnifying power of the telescope. Two such lenses shut up in a tube, make what is called the astronomical telescope. In looking at the heavens, the inversion of the picture causes no inconvenience. In the Galilean telescope (fig. 2) - So called from Galileo a concave eyepiece is placed behind the po
Fig. 2. sition of the picture, which lodges it at once in the eye. There is no inversion in this telescope.