THE PRINCIPLES INVOLVED IN THE CONSTRUCTION OF THE TELESCOPE. By THOMAS NOLAN, B. S. Written for VAN NOSTRAND'S ENGINEERING MAGAZINE. I THE OPTICAL PRINCIPLES INVOLVED IN The Telescope is certainly one of the my purpose in this dissertation to enter noblest monuments of human genius, into the discussion of the question as to and its invention will always be con- who was the inventor of the first "opticksidered as among the most remarkable tube," or to describe minutely and hisin the whole circle of human knowledge. torically all the several successive It is a work, in which, by following un- changes and improvements which it has consciously the plan of nature in the undergone in the hands of different formation of the eye, we have come the philosophers; but rather, to examine the nearest to the construction of a new theory of those forms of refracting and organ of sense, and by means of which, reflecting telescopes which have been of we have extended our views and re- more general application. searches far beyond the limits of our own globe-that sphere which nature seemed to have designed for our inquiries. Enabling us to penetrate into A telescope, in general, consists of a the immensity of space, and to become, tube containing a system of glass lenses, as it were, familiar with other worlds or a speculum in combination with such placed at almost immeasurable distances lenses; and is used to render distant obfrom us, the telescope has revealed an jects more clearly visible; (1) by enlarginfinitude of celestial bodies, whose ex-ing their apparent angular dimensions, istence must forever have remained un- and (2) by introducing into the eye a known to us, but for its invention. Its superior quantity of their light. history, like that of every complicated instrument, has been a history of improvements. The question of the origin of the invention, although abundantly inquired into, has never met with a satisfactory answer; and the question-who made the first telescope? will probably never be conclusively settled. For the invention, in its original form, we are indebted to accident, or to the trials of men who had little knowledge of the principles of the science upon which they were conferring so great a favor. I. THE SIMPLE REFRACTING TELESCOPE. Not a single thing, but a combination of things, the telescope in its earliest forms. In exhibiting the principles on which was a simple combination of certain the refracting telescope is constructed, kinds of convex and concave lenses, we must first explain the formation of known and used as spectacles many the image of an object at the focus of a years before; and whether the credit of the invention should be given to Metius, Lipperhey, Jansen, Baptista Porta, Galileo, or to others whose names are unknown, it is a most difficult and invidious task to decide. But interesting though these inquiries may be, it is not Those constructed with glass lenses only, are called dioptric or refracting, and the others catoptric or reflecting telescopes. In the refracting telescope, rays of light coming from the object are made to converge by a convex lens, and, if not intercepted, form an image at its focus. In the reflecting telescope, the image is formed by the reflection of the rays which impinge upon upon the concave surface of a speculum. We will first consider, lens. When a convex lens is placed before an object, an image of the latter is formed at a certain distance behind or before the refracting surface, whose magnitude is greater or less, according as it is formed farther from, or nearer to the convert this parallelism into a slight divergence or convergence, to suit the eyes of near or long-sighted persons. the top and bottom of the object AB | chanism itself shall be adjustable, so as, are represented separately. The ray Aa, by a movement of the parts inter se, to passing through the center of the lens O, is unaffected, because the surfaces through which it passes are parallel to each other, and from the property of the lens, all other rays from A, on passing through it, are brought to a focus somewhere on Aa, depending upon the curvature of the lens. In like manner, all rays from B are brought to a focus at b, each point of AB having its correspond The third condition is, that the pencils of parallel rays coming iuto the eye from different points of the object, shall be inclined to each other at greater angles than those actually subtended at the eye, by the respective intervals between the points themselves. The number ex Fig. 1. A F ing focus in ab. The latter is smaller than AB in proportion as cO is less than CO; and if we increase the focal length of Ó till ab is twice the distance away, it becomes double its present size. The place of an image depends upon the distance of an object, and when the latter is considered at an infinite distance, as is the case with the heavenly bodies, the image is formed at the principal focus of the lens, or the focus of parallel ray. If rays ABC &c., pass through a concave lens, as in (Fig. 2), they are not brought to a focus, but diverge as if they B pressing this ratio is the measure of the magnifying power of the telescope, and the apparent linear dimensions of an object being in proportion to its magnifying power, the apparent enlargement of its superficial area is as the square of the magnifying power. The simplest construction of the refracting telescope is that in which the image formed in the focus of a convex lens, or object-glass, is viewed through a second lens, or eye-glass, so placed as to have the image in its focus for parallel rays incident in the contrary direction, pupil is small, and since it involuntarily contracts in proportion to the quantity of light impinging upon the eye, the field of view is so much the less as the focus of PQ is greater. (3.) Since the nature of light will not admit of an eye-glass whose focus is diminished beyond a certain limit, and since the focus ought to be longer in proportion to the length of the focus of the object-glass, the field of view will be less the greater the length of the instrument, which inconvenience, together Fig. 3. 1. THE GALILEAN TELESCOPE. In this instrument, the eye-glass PQ (Fig. 3) is placed between the object-glass MN and its focus O', so that the axes of both glasses are in the same line AO', and their foci in the same point O'. (1.) The object OB being supposed infinitely distant, incident parallel rays, as AD, A'D', A"D", are first rendered converging towards O' by MN, and afterwards parallel by PQ. They are also much denser than before their first refraction, and, when received by the eye, paint an image of the object, so much the more P M with those above described, render this instrument almost useless for purposes of astronomical observation. (4.) The pencil of rays c, from B, under the axis, meets the eye in the direction cF, answering to a point also below the axis; and in the same manner, a ray issuing from any point of the object, above the axis, has a similar direction on emerging from the eyeglass. Consequently, objects and their images have like positions with respect to direction, when viewed through a telescope of this construction. vivid, as the density of the pencil of rays is then greater than before it fell upon MN. (5.) To determine the field of view, we let MN (Fig. 4) be the diameter of the object-glass, AB that of the pupil of (2.) The point B sends forth parallel the eye whose center is in the axis of the rays as CD and its two parallels, which telescope, and join M and B, the oppoare refracted towards some point b, and site extremities of these diameters, and then rendered parallel by PQ. Since let MB meet the axis in x, and the this pencil, in emerging from PQ, diverges from that formed by the point O, a greater number of the parts of the object will be seen as the eye is placed nearer to PQ, and as the pupil is more dilated. But since the dilation of the image in p. We draw also LB and Lp, and suppose pL and qL to be produced till they meet the object in P and Q. PM is refracted to the pupil in the direction MB, but every other ray in the pencil, as PL, and every ray which That is, the linear magnitude of the field | AF as the curvature of PQ is greater; so of view is measured by the angle which that the axis of the pencil which they the diameter of the pupil subtends at the center of the object-glass, increased by the difference between the angles which the diameter of the object-glass subtends at the pupil and at the image. form, cuts the common axis of the two lenses in F, the focus of PQ. Consequently, to see all of ob at the same time, the eye must be placed at F, the common intersection of all rays emitted from each point of ob, or of OB. (4.) b, the image of B, is refracted to the eye in the direction PF, and has a different direction from that in which it is emitted from B; and the ray which renders visible a point above the axis object-glass that their foci coincide in the axis of the telescope between the two glasses, instead of being beyond the eye-glass as in the Galilean telescope. PQ is the eye-glass, MN the objectglass, KD the axis, and o the common focus. (1.) The rays AD, A'D', A"D" from O in the object OB, infinitely distant, are refracted to the focus o where they form an image of O. (2.) This image o is considered as an object placed in the focus PQ, and rays reaches the eye as if it proceeded from below the axis; and thus the entire image is reversed. (5.) To find the field of view, we let MN (Fig. 6) be the diameter of the object-glass, AB that of the eye-glass, and draw NB, letting it cut the image pqr in p. We draw also pL, and conceive pL and EL to be produced until they meet the object in P and Q; and draw pE and NP, and also BO parallel to pE. Then the eye at O receives the ray NBO, which comes from a point in the object at the greatest visible distance from the an infinitely distant object, the parallel axis of the telescope. For rays from rays from Ò, in the axis produced, from any point above P, converge to a point an image in the focus o, whence, falling below p, and fall below AB; consequent- on, PQ they emerge parallel to each ly P is the exteme visible point in the other and to the axis. Falling upon KS, object, and QP is half the linear magni- they are refracted to the focus o', where tude of the visible area. a second image is formed, and then diverging, and falling on TV, they are refracted to the eye in directions parallel to the axis. In like manner, rays from B form in b an image of that point, whence, falling in PQ they are rendered parallel to each other, but oblique to the axis till they meet RS, by which they refracted to the second focus b'; whence, falling on TV, they are again rendered (6.) To find the brightest area, we draw MB cutting pqr in s, and the axis LE in x', and draw Ls, and conceive it produced till it meet the object in S. Then, (a) if x'E be greater than qE, all rays from S falling upon MN, are refracted to AB; for, SM is refracted in the direction MSB, and any other ray of the pencil, as SL, crosses MB at s, and falls somewhere between A and B. In like manner, rays from any point between S and Q, are refracted to AB. (b) If and coincide, the brightness of the field increases to the center. (c) If q fall between Land x', all the rays belong ing to any one pencil incident upon MN, are not received by AB. 3. THE TERRESTRIAL TELESCOPE. The nature of this construction is easily comprehended by reference to (Fig. 7). The four lenses have one common axis A f. and each contiguous two are so situated that their foci coincide. If OB be parallel to each other, but so inclined to the axis as to meet it again with all the other rays in the focus f, where the eye is placed to receive the final impression, which is that of an image corresponding, as to its direction, with the object itself. For the ray b'Vf, which carries the image of the point B, has a similar direction with respect to the axis, as the ray proceeding immediately from B. As the properties of this instrument are analogous to those of the astronomical telescope, it is unnecessary to enter into further discussion of the principles of its construction. FILTRATION THROUGH SPONGY IRON. From "Engineering." It is now some years since Mr. Gustav | Pollution, in the reports of the RegisBischof demonstrated the remarkable trar General, in the Army Medical Reproperties of spongy iron as a filtering port for 1877, and elsewhere, the mamaterial for the purification of water for terial has not yet been employed on a drinking purposes, and spongy iron filt-large scale for the filtration of water at ers are already in extensive use for do- any of our water works, a fact which is mestic purposes. Notwithstanding this, to be regretted when the remarkable efhowever, and notwithstanding also the fect of the material on Thames water is testimony to the powers of spongy iron taken into consideration. which has been afforded in the sixth report of the Royal Commission on Rivers Although, however, spongy iron has not yet been introduced at water works |