“How then shall I attempt to sing ofHimWho, light himself, in uncreated lightInvested deep, dwells awfully retiredFrom mortal eye or angel’s purer ken;Whose single smile has, from the first of time,Filled, overflowing, all yon lamps of heaven,That beam for ever through the boundless sky.”—Thomson.
“How then shall I attempt to sing ofHimWho, light himself, in uncreated lightInvested deep, dwells awfully retiredFrom mortal eye or angel’s purer ken;Whose single smile has, from the first of time,Filled, overflowing, all yon lamps of heaven,That beam for ever through the boundless sky.”—Thomson.
“How then shall I attempt to sing ofHimWho, light himself, in uncreated lightInvested deep, dwells awfully retiredFrom mortal eye or angel’s purer ken;Whose single smile has, from the first of time,Filled, overflowing, all yon lamps of heaven,That beam for ever through the boundless sky.”—Thomson.
The eye is the instrument by which we perceive the beautiful and multifarious effects of this universal agent. Its delicate and complicated structure, its diversified muscles, its coats and membranes, its different humours possessed of different refractive powers, and the various contrivances for performing and regulating its external and internal motions, so as to accomplish the ends intended—clearly demonstrate this organ to be a master-piece of Divine mechanism—the workmanship of Him whose intelligence surpasses conception, and whose Wisdom is unsearchable. ‘Our sight (says Addison) is the most perfect and delightful of all our senses. It fills the mind with the largest variety of ideas, converses with its objects at the greatest distance, and continues the longest in action, without being tired or satiated with its proper enjoyments. The sense of feeling can indeed give us a notion of extension, shape, and all other ideas that enter the eye, except colours; but at the same time it is very much strained, and confined in its operation to the number, bulk and distance of its particular objects. Our sight seems designed to supply all these defects, and may be considered as a more delicate and diffusive kind of touch that spreads itself over an infinite multitude of bodies, comprehends the largest figures, and brings into our reach some of the more remote parts of the universe.’
Could we suppose an order of beings endued with every human faculty but that of sight, it would appear incredible to such beings—accustomed only to the slow information of touch—that by the addition of an organ consisting of a ball and socket, of an inch diameter, they might be enabled, in an instant of time, without changing their place, to perceive the disposition of a wholearmy, the order of a battle, the figure of a magnificent palace, or all the variety of a landscape. If a man were by feeling to find out the figure of the Peak of Teneriffe, or even of St. Peter’s church at Rome, it would be the work of a lifetime. It would appear still more incredible to such beings as we have supposed, if they were informed of the discoveries which may be made by this little organ in things far beyond the reach of any other sense—that, by means of it we can find our way in the pathless ocean—that we can traverse the globe of the earth, determine its figure and dimensions, and delineate every region of it—yea, that we can measure the planetary orbs, and make discoveries in the sphere of the fixed stars. And, if they were farther informed that, by means of this same organ, we can perceive the tempers and dispositions, the passions and affections of our fellow-creatures, even when they want most to conceal them—that when the tongue is taught most artfully to lie and dissemble, the hypocrisy should appear in the countenance to a discerning eye—and that by this organ we can often perceive what is straight and what is crooked in the mind as well as in the body—would it not appear still more astonishing to beings such as we have now supposed?33
Notwithstanding these wonderful properties of the organ of vision, the eye, when unassisted by art, is comparatively limited in the range of its powers. It cannot ascertain the existence of certain objects at the distance of three or four miles, nor perceive what is going forward in nature or art beyond such a limit. By its natural powers we perceive the moon to be a globe about half adegree in diameter, and diversified with two or three dusky spots, and that the sun is a luminous body of apparently the same size—that the planets are luminous points, and that about a thousand stars exist in the visible canopy of the sky. But the ten thousandth part of those luminaries, which are within the reach of human vision, can never be seen by the unassisted eye. Here theTELESCOPEinterposes, and adds a new power to the organ of vision, by which it is enabled to extend its views to regions of space immeasurably distant, and to objects, the number and magnitude of which could never otherwise have been surmised by the human imagination. By its aid we obtain a sensible demonstration that space is boundless—that the universe is replenished with innumerable suns and worlds—that the remotest regions of immensity, immeasurably beyond the limits of unassisted vision, display the energies of Creating Power, and that the Empire of the Creator extends far beyond what eye hath seen or the human imagination can conceive.
The telescope is an instrument of a much more wonderful nature than what most people are apt to imagine. However popular such instruments now are, and however common a circumstance it is to contemplate objects at a great distance which the naked eye cannot discern, yet, prior to their invention and improvement, it would have appeared a thing most mysterious, if not impossible, that objects at the distance of ten miles could be made to appear as if within a few yards of us, and that some of the heavenly bodies could be seen as distinctly as if we had been transported by some superior power, hundreds of millions of miles beyond the bounds of our terrestrial habitation. Who could ever have imagined—reasoningapriori—that the refraction of light in glass—the same power by which a straight rod appears crooked in water, by which vision is variously distorted, and by which we are liable to innumerable deceptions—that that same power, or law of nature, by the operation of which the objects in a landscape appear distorted when seen through certain panes of glass in our windows, that that power should ever be so modified and directed as to extend the boundaries of vision, and enable us clearly to distinguish scenes and objects at a distance a thousand times beyond the natural limits of our visual organs? Yet such are the discoveries which science has achieved, such the powers it has brought to light, that by glasses ground into different forms, and properly adapted to each other, we are enabled as it were to contract the boundaries of space, to penetrate into the most distant regions, and to bring within the reach of our knowledge the most sublime objects in the universe.
When Pliny declared in reference to Hipparchus, the ancient astronomer, ‘Ausus rem Deo improbam annumerare posteris stellas,’—that ‘he dared to enumerate the stars for posterity, an undertaking forbidden by God,’ what would that natural historian have said, had it been foretold that in less than 1600 years afterwards, a man would arise who should enable posterity to perceive, and to enumerate ten times more new stars than Hipparchus ever beheld—who should point out higher mountains on the moon than on the earth, who should discover dark spots, as large as our globe, in the sun, the fountain of light—who should descry four moons revolving in different periods of time around the planet Jupiter, and could show to surrounding senators the varying phases of Venus? and that another would soon after arise who shouldpoint out a double ring of six hundred thousand miles in circumference, revolving around the planet Saturn, and ten hundreds of thousands of stars which neither Hipparchus nor any of the ancient astronomers could ever descry? Yet these are only a small portion of the discoveries made by Galileo and Herschel, by means of the telescope. Had any one prophetically informed Archimedes, the celebrated geometrician of Syracuse, that vision would, in after ages, be thus wonderfully assisted by art—and further, that one manner of improving vision would be to place a darkopakebody directly between the object and the eye—and that another method would be, not to look at the object, but to keep the eye quite in a different, and even in anoppositedirection, or to stand with the back directly opposed to it, and to behold all the parts of it, invisible to the naked eye, most distinctly in this way—he would, doubtless have considered the prophet as an enthusiastic fool or a raving madman. Yet these things have been realized in modern times in the fullest extent. In the Gregorian reflecting telescope an opake body, namely the small speculum near the end of the tube, interposesdirectlybetween the eye and the object. In the Newtonian Reflector, and in the diagonal eye-pieces formerly described, the eye is directed in a line at right angles to the object, or a deviation of 90 degrees from the direct line of vision. In Herschel’s’ large telescopes, and in theAerial Reflectorformerly described (in pp. 311-325) the back is turned to the object, and the eye in an opposite direction.
These circumstances should teach us humility and a becoming diffidence in our own powers; and they should admonish us not to be too dogmatical or peremptory in affirming what is possibleor impossible in regard either to nature or art, or to the operations of the Divine Being. Art has accomplished, in modern times, achievements, in regard to locomotion, marine and aërial navigation, the improvement of vision, the separation and combinations of invisible gases, and numerous other objects, of which the men of former ages could not have formed the least conception. And even yet, we can set no boundaries to the future discoveries of science and the improvements of art; but have every reason to indulge the hope that, in the ages to come, scenes of Divine mechanism in the system of nature will be unfolded, and the effects of chemical and mechanical powers displayed, of which the human mind, in its present state of progress, cannot form the most imperfect idea. Such circumstances likewise should teach us not to reject any intimations which have been made to us in relation to the character, attributes, and dispensations of the Divine Being, and the moral revelations of his will given in the Sacred Records, because we are unable to comprehend every truth and to remove every difficulty, which relates to the moral government of the Great Ruler of the universe. For, if we meet with many circumstances in secular science, and even in the common operations of nature, which are difficult to comprehend—if even the construction of such telescopes as we now use, would have appeared an incomprehensible mystery to ancient philosophers—we must expect to find difficulties almost insurmountable to such limited minds as ours, in the eternal plans and moral arrangements of the “King Immortal and Invisible,” as delineated only in their outlines, in the Sacred Oracles—particularly those which relate to the origin of physical and moral evil, theultimate destiny of man, and the invisible realities of a future world.
TheUTILITYof the telescope may be considered in relation to the following circumstances.
In the first place, it may be considered as an instrument or machine which virtually transports us to the distant regions of space. When we look at the moon through a telescope which magnifies 200 times, and survey its extensive plains, its lofty peaks, its circular ranges of mountains, throwing their deep shadows over the vales, its deep and rugged caverns, and all the other varieties which appear on the Lunar surface, we behold such objects in the same manner as if we were standing at a point 238,800 miles from the earth in the direction of the moon, or only twelve hundred miles from that orb, reckoning its distance to be 240,000 miles. When we view the planet Saturn with a similar instrument, and obtain a view of its belts, and satellites, and its magnificent rings, we are transported, as it were, through regions of space, to a point in the heavens more thannine hundred millions of milesfrom the surface of our globe, and contemplate those august objects, as if we were placed within five millions of miles of the surface of that planet.34Although a supernatural power, sufficient to carry us in such a celestial journey, a thousand miles every day, were exerted—it would require more than two thousand four hundred and sixty years, before we could arrive atsuch a distant position; yet the telescope, in a few moments, transports our visual powers to that far distant point of space. When we view, with such an instrument, the minute and very distant clusters of stars in the Milky Way, we are carried in effect through the regions of space to the distance offive hundred thousand millions of milesfrom the earth; for we behold those luminaries through the telescope nearly as if they were actually viewed from such a distant point in the spaces of the firmament. These stars cannot be conceived as less thana hundred billionsof miles from our globe, and the instrument we have supposed brings them within the two hundredth part of this distance. Suppose we were carried forward by a rapid motion towards this point, at the rate of a thousand milesevery hour, it would require more thanfifty-seven thousand years, before we could reach that very distant station in space to which the telescope,in effect, transports us. So that this instrument is far more efficient in opening to our view the scenes of the universe than if we were invested with powers of locomotion to carry us through the regions of space, with the rapidity of a cannon ball at its utmost velocity; and all the while we may sit at ease in our terrestrial apartments.
In the next place, the telescope has beenthe means of enlarging our views of the sublime scenes of creation, more than any other instrument which art has contrived. Before the invention of this instrument the universe was generally conceived as circumscribed within very narrow limits. The earth was considered as among the largest bodies in creation; the planets were viewed as bodies of a far less size than what they are now found to be; no bodies similar to ourmoon were suspected as revolving around any of them; and the stars were supposed to be little more than a number of brilliant lamps hung up to emit a few glimmering rays, and to adorn the canopy of our earthly habitation. Such a wonderful phenomenon as the Ring of Saturn was never once suspected, and the sun was considered as only a large ball of fire. It was suspected, indeed, that the moon was diversified with mountains and vales, and that it might possibly be a habitable world; but nothing certainly could be determined on this point, on account of the limited nature of unassisted vision. But the telescope has been the means of expanding our views of the august scenes of creation to an almost unlimited extent. It has withdrawn the veil which formerly interposed to intercept our view of the distant glories of the sky. It has brought to light five new planetary bodies, unknown to former astronomers, one of which is more than eighty times larger than the earth—and seventeensecondaryplanets which revolve around the primary. It has expanded the dimensions of the solar system to double the extent which was formerly supposed. It has enabled us to descry hundreds of comets which would otherwise have escaped our unassisted vision, and to determine some of their trajectories and periods of revolution.
It has explored the profundities of the Milky Way, and enabled us to perceive hundreds of thousands of those splendid orbs, where scarcely one is visible to the naked eye. It has laid open to our view thousands ofNebulæ, of various descriptions, dispersed through different regions of the firmament—many of them containing thousands of separate stars. It has directed our investigations to thousands of double, treble and multiplestars—suns revolving around suns, and systems around systems, and has enabled us to determine some of the periods of their revolutions. It has demonstrated the immense distances of the starry orbs from our globe, and their consequent magnitudes; since it shows us that, having brought them nearer to our view by several hundreds or thousands of times, they still appear only as so many shiningpoints. It has enabled us to perceive that mighty changes are going forward throughout the regions of immensity—new stars appearing, and others removed from our view, and motions of incomprehensible velocity carrying forward those magnificent orbs through the spaces of the firmament. In short, it has opened a vista to regions of space so immeasurably distant, that a cannon ball impelled with its greatest velocity, would not reach tracts of creation so remote in two thousand millions of years, and even light itself, the swiftest body in nature, would require more than a thousand years before it could traverse this mighty interval. It has thus laid a foundation for our acquiring an approximate idea of the infinity of space, and for obtaining a glimpse of the far distant scenes of creation, and the immense extent of the universe.
Again, the telescope, in consequence of the discoveries it has enabled us to make, has tendedto amplify our conceptions of the attributes and the Empire of the Deity. The amplitude of our conceptions of the Divine Being bears a certain proportion to the expansion of our views in regard to his works of creation, and the operations he is incessantly carrying forward throughout the universe. If our views of the works of God, and of the manifestations he has given of himself to his intelligent creatures, be circumscribed to anarrow sphere, as to a parish, a province, a kingdom, or a single world, our conceptions of that Great Being, will be proportionably limited. For it is chiefly from the manifestation of God in the material creation that our ideas of his Power, his Wisdom, and his other natural attributes, are derived. But in proportion to the ample range of prospect we are enabled to take of the operations of the Most High, will be our conceptions of his character, attributes, and agency. Now, the telescope—more than any other invention of man—has tended to open to our view the most magnificent and extensive prospects of the works of God. It has led us to ascertain that, within the limits of the solar system, there are bodies which, taken together, comprise a mass of matter nearly two thousand five hundred times greater than that of the earth—that these bodies are all constituted and arranged in such a manner as to fit them for being habitable worlds—and that the sun, the centre of this system, is five hundred times larger than the whole. But, far beyond the limits of this system, it has presented to our view a universe beyond the grasp of finite intelligences, and to which human imagination can assign no boundaries. It has enabled us to descry suns clustering behind suns, rising to view in boundless perspective, in proportion to the extent of its magnifying and illuminating powers—the numbers of which are to be estimated, not merely by thousands, and tens of thousands, and hundreds of thousands, but by scores ofmillions—leaving us no room to doubt that hundreds of millions more, beyond the utmost limits of human vision, even when assisted by art, lie hid from mortal view’s in the unexplored and unexplorable regions of immensity.
Here, then, we are presented with a scene which gives us a display ofOmnipotent Powerwhich no other objects can unfold, and which, without the aid of the telescope, we should never have beheld—a scene which expands our conceptions of the Divine Being, to an extent which the men of former generations could never have anticipated—a scene which enables us to form an approximate idea of Him who is the “King Eternal, Immortal, and Invisible,” who “created all worlds, and for whose pleasure they are, and were created.” Here we behold the operations of a Being whose power is illimitable and uncontrollable, and which far transcends the comprehension of the highest created intelligences—a power, displayed not only in the vast extension of material existence, and the countless number of mighty globes which the universe contains—but in the astonishinglyrapid motionswith which myriads of them are carried along through the immeasurable spaces of creation,—some of those magnificent orbs moving with a velocity of one hundred and seventy thousand miles an hour. Here, likewise, we have a display of the infiniteWisdomand Intelligence of the Divine Mind, in the harmony and order with which all the mighty movements of the universe are conducted—in proportionating the magnitudes, motions and distances of the planetary worlds—in the nice adjustment of the projectile velocity to the attractive power—in the constant proportion between the times of the periodical revolution of the planets and the cubes of their mean distances—in thedistancesof the several planets from the central body of the system, compared with their respectivedensities—and in the constancy and regularity of their motions, and the exactness with which they accomplish their destined rounds—allwhich circumstances evidently show that He who contrived the universe is “the only Wise God,” who is “wonderful in counsel and excellent in working.” Here, in fine, is a display ofboundless benevolence. For we cannot suppose, for a moment, that so many myriads of magnificent globes, fitted to be the centres of a countless number of mighty worlds, should be nothing else than barren wastes, without the least relation to intelligent existence. And if they are peopled with intellectual beings of various orders—how vast must be their numbers, and how overflowing that Divine Beneficence which has provided for them all, every thing requisite to their existence and happiness!
In these discoveries of the telescope, we obtain a glimpse of the grandeur and the unlimited extent of God’s universal empire. To this empire no boundaries can be perceived. The larger, and the more powerful our telescopes are, the further are we enabled to penetrate into those distant and unknown regions; and however far we penetrate into the abyss of space, new objects of wonder and magnificence still continue rising to our view—affording the strongest presumption, that were we to penetrate ten thousand times farther into those remote spaces of immensity, new suns, and systems, and worlds would be disclosed to our view. Over all this vast assemblage of material existence, and over all the sensitive and intellectual beings it contains, God eternally and unchangably presides; and the minutest movements, either of the physical or the intelligent system, throughout every department of those vast dominions, are at every moment “naked and open” to his Omniscient eye. Whatboundless Intelligenceis implied in theSuperintendenceandarrangementof the affairs of such an unlimited empire! and what a lofty and expansive idea does it convey of Him who sits on the throne of Universal Nature, and whose greatness is unsearchable! But without the aids of the telescopic tube, we could not have formed such ample conceptions of the greatness, either of the Eternal Creator himself, or of the universe which he hath brought into existence.
Besides the above, the following uses of the telescope, in relation to science and common life, may be shortly noticed:—
In the business of astronomy, scarcely any thing can be done with accuracy without the assistance of the telescope. 1. It enables the astronomer to determine with precisionthe transits of the planets and stars, across the meridian; and on the accuracy with which these transits are obtained, a variety of important conclusions and calculations depend. The computation of astronomical and nautical tables for aiding the navigator in his voyages round the globe, and facilitating his calculations of latitude and longitude, is derived from observations made by the telescope, without the use of which instrument, they cannot be made with precision. 2. Theapparent diameters of the planetscan only be measured by means of this instrument, furnished with a micrometer. By the naked eye no accurate measurements of the diameters of these bodies can be taken; and without knowing their apparent diameters, in minutes or seconds, their real bulk cannot be determined, even although their exact distances be known. The differences, too, between their polar and equatorial diameters cannot be ascertained without observations made by powerful telescopes. For example, the equatorial diameter of Jupiter is found to be in proportion to the polar as 14to 13, that is, the equatorial is more than 6000 miles longer than the polar diameter, which could never have been determined by observations made by the naked eye. 3. Theparallaxesof the heavenly bodies can only be accurately ascertained by the telescope; and it is only from the knowledge of their parallaxes, that their distances from the earth or from the sun can be determined. In the case of the fixed stars, nothing of the nature of a parallax could ever be expected to be found without the aid of a telescope. It was by searching for the parallax of a certain fixed star, that the important fact of theAberration of lightwas discovered. The observations, for this purpose, were made by means of a telescope 24 feet long, fixed in a certain position. 4. The motions and revolutionary periods ofSidereal systems, can only be determined by observations made by telescopes of great magnifying and illuminating powers. Without a telescope the small stars which accompany double or treble stars cannot be perceived, and much less their motions or variation of their relative positions. Before the invention of the telescope such phenomena—now deemed so wonderful and interesting—could never have been surmised. 5. The accurate determination of the longitude of places on the earth’s surface is ascertained by the telescope, by observing with this instrument the immersions and emersions of the satellites of Jupiter. From such observations, with the aid of a chronometer, and having the time at any known place, the situation of any unknown place is easily determined. But the eclipses of Jupiter’s moons can be perceived only by telescopic instruments of considerable power. 6. By means of a telescope, with cross hairs in the focus of the eye-glass, and attached to a Quadrant, the altitude of the sun or of a star, particularlythe pole-star, may be most accurately taken; and, from such observations, thelatitudeof the place may be readily and accurately deduced.
Again, in theSurveying of land, the telescope is particularly useful; and for this purpose it is mounted on a stand with a horizontal and vertical motion, pointing out by divisions the degrees and minutes of inclination of the instrument. For the more accurate reading of these divisions, the two limbs are furnished with a Nonius, orVernier’s scale. The object here is to take the angular distances between distant objects on a plane truly horizontal; or else the angular elevation or depression of objects above or below the plane of the horizon. In order to obtain either of those kinds of angles to a requisite degree of exactness, it is necessary that the surveyor should have as clear and distinct a view as possible of the objects, or station-staves, which he fixes up for his purpose, that he may with the greater certainty determine the point of the object which exactly corresponds with the line he is taking. Now, as such objects are generally at too great a distance for the surveyor to be able to distinguish with the naked eye, he takes the assistance of the telescope, by which he obtains, 1. A distinct view of the object to which his attention is directed, and 2. he is enabled to determine the precise point of the object aimed at, by means of the cross hairs in the focus of the eye-glass. A telescope mounted for this purpose is called aTheodolite, which is derived from two Greek words θεομαιto see, and οδος,the wayordistance.
In the next place, the telescope is an instrument of special importance, in the conducting ofTelegraphs, and in the conveyance ofsignalsof all descriptions. Without its assistance telegraphicdispatches could not be conveyed with accuracy to any considerable distance, nor in quadruple the time in which they are now communicated, and the different stations would need to be exceedingly numerous. But by the assistance of the telescope information may be communicated, by a series of telegraphs, with great rapidity. Twenty-seven telegraphs convey information from Paris to Calais—a distance of 160 miles—in 3 minutes; twenty-two from Paris to Lisle in 2 minutes; forty-six from Strasburg to Paris in 4½ minutes; and eighty from Paris to Brest in 10 minutes. In many other cases which occur both on land and on sea, the telescope is essentially requisite for descrying signals. TheBell-Rock Light House, for example, is situated 12 miles from Arbroath, and from every other portion of land, so that the naked eye could not discern any signal which the keepers of that light could have it in their power to make; but by means of a large telescope in the station-house in Arbroath, the hoisting of a ball every morning at 9A.M.—which indicates that ‘All is well’—may be distinctly recognised.
Many other uses of this instrument, in the ordinary transactions of life, will readily occur to the reader; and therefore I shall only mention the following purpose to which it may be applied, namely,—
To measure the distance of an object from one station.This depends upon the increase of the focal distance of the telescope in the case of near objects. Look through a telescope at the object whose distance is required, and adjust the focus till it appear quite distinct; then slide in the drawer, till the object begins to be obscure, and mark that place of the tube precisely. Nextdraw out the tube till the object begins to be again obscured, and then make another mark as before. Then take the middle point between these two marks, and that will be the point where the image of the object is formed most distinctly; which is to be nicely measured from the object lens, and compared with the solar focus of the lens or telescope, so as to ascertain their difference. And the rule for finding the distance is,—‘As the difference between the focal distance of the object, and the solar focal distance : Is to the solar focal distance :: So is the focal distance of the object : To its true distance from the object lens.’ An example will render this matter more perspicuous.
figure 84.
figure 84.
Let AB (fig. 84.) be the object lens, EY the eye-glass, FC the radius, or focus of the lens AB, and Cfthe focal distance of the object OB, whose distance is to be measured. Now suppose CF = 48 inches, or 4 feet, and that we find by the above method that Cfis 50 inches, then Ffis 2 inches; and the analogy is:—As Ff= 2, is to CF = 48, so is Cf= 50, to CQ = 1200 inches, or 100 feet. Again, suppose Cf= 49 inches, then will Ff= 1 inch; and the proportion is, 1 : 48 :: 49 : 2352 = QC, or 196 feet. A telescope of this focal length, however, will measure only small distances. But, suppose AB a lenswhose solar focus is 12 feet, or 144 inches; and that we find, by the above method, that Cf, or the focal distance of the object, is 146 inches; then will Ffbe 2 inches, and the proportion will be, as 2 : 144 :: 146 : 21024 inches, or 1752 feet = the distance QC. If with such a large telescope, we view an object OB, and find Ffbut1/10th of an inch, this will give the distance of the object as 17292 feet or nearly 3⅓ miles.
Since the difference between the radius of the object lens and the focal distance of the object is so considerable as 2 inches in a tube of 4 feet, and more than 12 inches in one of 12 feet, a method might be contrived for determining the distance of near objects by the former, and more distant objects by the latter, by inspection only. This may be done by adjusting or drawing a spiral line round the drawer or tube, through thetwo inch spacein the small telescope, and by calculation, graduate it for every 100 feet, and the intermediate inches, and then, at the same time we view an object, we may see its distance on the tube. In making such experiments, a common object-glass of a long focal length, and a single eye-glass, are all that is requisite; since the inverted appearance of the object can cause no great inconveniency.
I originally intended to enter into particular details on this subject, for the purpose of gratifying those mechanics and others who wish to amuse themselves by constructing telescopes and other optical instruments for their own use; but, having dwelt so long on the subject of telescopes, in the preceding pages, I am constrained to confine myself to a very general sketch.
1.To grind and polish lenses for eye-glasses, microscopes, &c.
First provide an upright spindle, at the bottom of which a pulley is fixed, which must be turned by a wheel by means of a cord and handle. At the top of the spindle make a screw the same as a lathe-spindle, on which you may screw chocks of different sizes, to which the brass tool in which the lens is to be ground, may be fixed. Having fixed upon the breadth and focal length of the lens, and whether it is to be a plano, or a double convex—take a piece of tin-plate or sheet copper, and, with a pair of compasses, draw an arch upon its surface, near one of its extremities, with a radius equal to the focal distance of the lens, ifintended to be double convex, or with half that distance, if it is to be plano-convex. Remove with a file that part of the copper which is without the circular arch, and then aconvexgage is formed. With the same radius strike another arch, and having removed that part of the copper which iswithinit, aconcavegage will be obtained. The brass tool, in which the glass is to be ground, is then to be fixed upon a turning-lathe, and turned into a portion of a concave sphere, so as to correspond to the convex gage. In order to obtain an accurate figure to the concave tool, a convex tool of exactly the same radius is generally formed, and they are ground one upon another with flour emery; and when they exactly coincide, they are fit for use. The convex tool will serve for grindingconcaveglasses of the same radius—and it should be occasionally ground in the concave tool to prevent it from altering its figure.
The next thing to be attended to is, to prepare the piece of glass which is to be ground, by chipping it in a circular shape, by means of a large pair of scissors, and removing the roughness from its edges by a common grind-stone. The faces of the glass near the edges should likewise be ground on the grind-stone, till they nearly fit the concave gage, by which the labour of grinding in the tool will be considerably saved. The next thing required is to prepare the emery for grinding, which is done in the following manner. Provide four or five clean earthen vessels; fill one of them with water, and put into it a pound or half a pound of fine emery, and stir it about with a stick; after which let it stand 3 or 4 seconds, and then pour it into another vessel, which may stand about 10 seconds; then pour it off again into the severalvessels till the water is quite clear; and by this means, emery of different degrees of fineness is obtained, which must be kept separate from each other, and worked in their proper order, beginning at the first, and working off all the marks of the grind-stone; then take of the second, next of the third, &c.,—holding the glass upon the pan or tool with a light hand, when it comes to be nearly fit for polishing. The glass in this operation should be cemented to a wooden handle, by means of pitch or other strong cement. After the finest emery has been used, the roughness which remains may be taken away, and a slight polish given by grinding the glass with pounded pumice-stone. Before proceeding to the polishing, the glass should be ground as smooth as possible, and all the scratches erased, otherwise the polishing will become a tedious process. The polishing is performed as follows: Tie a piece of linen rag or of fine cloth about the tool, and with fine putty, (calcined tin), or colcothar of vitriol (a very fine powder, sometimes called the red oxide of iron) moistened with water, continue the grinding motion, and in a short time there will be an excellent polish.
In order to grind lenses very accurately for the finest optical purposes, particularly object-glasses for telescopes—the concave tool is firmly fixed to a table or bench, and the glass wrought upon it by the hand with circular strokes so that its centre may never go beyond the edges of the tool. For every 6 or 7 circular strokes, the glass should receive 2 or 3 cross ones along the diameter of the tool, and in different directions; and while the operation is going on, the convex tool should, at the end of five minutes, be wrought upon the concave one for a few seconds, in order to preservethe same curvature to the tools and to the glass. The finest polish is generally given in the following way. Cover the concave tool with a layer of pitch hardened by the addition of a little rosin, to the thickness of1/15th of an inch. Then, having taken a piece of thin writing paper, press it upon the surface of the pitch with the convex tool, and pull the paper quickly from the pitch before it has adhered to it; and if the surface of the pitch is marked every where with the lines of the paper, it will be truly spherical. If any paper remains on the surface of the pitch, it may be rubbed off by soap and water, and if the marks of the paper should not appear on any part of it, the operation must be repeated, till the polisher or bed of pitch is accurately spherical. The glass is then to be wrought on the polisher by circular and cross strokes with the putty or colcothar, till it has received a complete polish. When one side is finished, the glass must be separated from its handle, by inserting the point of a knife between it and the pitch, and giving it a gentle stroke. The pitch which remains upon the glass may be removed by rubbing it with a little oil or spirits of wine. The operation of polishing on cloth is slower, and the polish less perfect than on pitch; but it is a mode best fitted for those who have little experience, and who would be apt, in the first instance, to injure the figure of the lens by polishing it on a bed of pitch.
2.On the method of casting and grinding the Specula of Reflecting Telescopes.
The first thing to be considered in the formation of reflecting telescopes, is thecompositionof the metal of which the specula are made. The qualities required are—a sound uniform metal, free from all microscopic pores—not liable to tarnishby absorption of moisture from the atmosphere—not so hard as to be incapable of taking a good figure and polish—nor so soft as to be easily scratched, and possessing a high reflecting power. Various compositions have been used for this purpose, of which the following are specimens:—Take good Swedish copper 32 ounces, and when melted, add 14½ ounces of grain tin to it; then, having taken off the scoria, cast it into an ingot. This metal must be a second time melted to cast a speculum; but it will fuse in this compound state with a small heat, and therefore will not calcine the tin to putty. It should be poured off as soon as it is melted, giving it no more heat than is absolutely necessary. The best method for giving the melted metal a good surface is this: the moment before it is poured off, throw into the crucible a spoonful of charcoal-dust; immediately after which the metal must be stirred with a wooden spatula and poured into the moulds.—The following is another composition somewhat similar. Take 2 parts copper as pure as it is possible to procure; this must be melted in a crucible by itself. Then put, in another crucible, 1 part of pure grain tin. When they are both melted, mix and stir them with a wooden spatula, keeping a good flux on the melted surface to prevent oxidation, and then pour the metal quickly into the moulds, which may be made of founder’s loam.
The composition suggested, more than half a century ago, by the Rev. Mr. Edwards, has often been referred to with peculiar approbation. This gentleman took a great deal of pains to discover the best composition, and to give his metals a fine polish and the true parabolical figure. His telescopes were tried by Dr. Maskelyne, the Astronomer Royal, who found them greatly to excel inbrightness, and to equal in other respects those made by the best artists. They showed a white object perfectly white, and all objects of their proper colour. He found, after trying various combinations, the following to be the best: namely 32 ounces of copper, with 15 or 16 ounces of grain tin, (according to the purity of the copper) with the addition of one ounce of brass, one of silver, and one ounce of arsenic. This, he affirms, will form a metal capable, when polished in a proper manner, of reflecting more light than any other metal yet made public.
The Rev. J. Little, in his observations on this subject in the ‘Irish Transactions,’ proposes the following composition, which he found to answer the purpose better than any he had tried, namely—32 parts of best bar copper, previously fluxed with the black flux, of two parts tartar and one of nitre, 4 parts brass, 16 parts tin, and 1¼ arsenic. If the metal be granulated, by pouring it, when first melted, into water, and then fused a second time, it will be less porous than at first. In this process, the chief object is, to hit on the exact point of the saturation of the copper, &c., by the tin. For, if the latter be added in too great quantity, the metal will be dull coloured and soft; if too little, it will not attain the most perfect whiteness, and will certainly tarnish.35
When the metal is cast, and prepared by the common grind-stone for receiving its proper figure—the gages and grinding-tools are to be formed in the same manner as formerly described for lenses, with this difference, that the radius of the gages must always bedoublethe focal length of the speculum, as the focus of parallel rays byreflection is at one half the radius of concavity. In addition to the concave and convex tools—which should be only a little broader than the metal itself—a convex elliptical tool of lead and tin should be formed with the same radius, so that its transverse should be to its conjugate diameter as 10 to 9, the latter being exactly equal to the diameter of the metal. The grinding of the speculum is then to be commenced, on this tool, with coarse emery powder and water, when the roughness is taken off, by moving the speculum across the tool, in different directions, walking round the post on which the tool is fixed, holding the speculum by the wooden handle to which it is cemented. It is then to be wrought with great care on the convex brass tool, with circular and cross strokes, and with emery of different degrees—the concave tool being sometimes ground upon the convex one, to keep them all of the same radius, and when every scratch is removed from its surface, it will be fit for receiving the final polish.
When the metal is ready for polishing, the elliptical tool is to be covered with black pitch about1/20th of an inch thick, and the polisher formed in the same way as in the case of lenses, either with the concave brass tool or with the metal itself. The colcothar of vitriol should then be triturated between two surfaces of glass, and a considerable quantity of it applied at first to the surface of the polisher. The speculum is then to be wrought, in the usual way, upon the polishing tool, till it has received a brilliant lustre, taking care to use no more of the colcothar, if it can be avoided, and only a small quantity of it, if it should be found necessary. When the metal moves stiffly on the polisher, and the colcotharassumes a dark muddy hue, the polish advances with great rapidity. The tool will then grow warm, and would probably stick to the speculum, if its motion were discontinued for a moment. At this stage of the process, therefore, we must proceed with great caution, breathing continually on the polisher, till the friction is so great, as to retard the motion of the speculum. When this happens, the metal is to be slipped off the tool at one side, cleaned with soft leather, and placed in a tube for the purpose of trying its performance; and if the polishing has been conducted with care, it will be found to have a trueparabolicfigure.36
It was formerly the practice, before the speculum was brought to the polisher, to smooth it on abed of hones, or a convex tool made of the best blue stone, such as clockmakers use in polishing their work, which was made one fourth part larger than the metal which was to be ground upon it, and turned as true as possible to a gage. But this tool is not generally considered as absolutely necessary, except when silver and brass enter into the composition of the metal, in order to remove the roughness which remains after grinding with the emery.
To try the figure of the metal.—In order to this, the speculum must be placed in the tube of the telescope for which it is intended; and, at about 20 or 30 yards distant, there should be put up a watch-paper, or similar object, on which there are some very fine strokes of an engraver. An annular kind of diagram should be made with card-paper, so as to cover a circular portion of the middle part of the speculum, between the hole and the circumference, equal in breadth to about1/8of its diameter. This paper ring should be fixed in the mouth of the telescope, and remain so during the whole experiment. There must likewise be two other circular pieces of card-paper cut out, of such sizes, that one may cover the centre of the metal, by completely filling the hole in the annular piece now described: and the other such a round piece as shall exactly fill the tube, and so broad as that the inner edge just touches the outward circumference of the middle annular piece. All these pieces together will completely shut up the mouth of the telescope. Let the round piece which covers the centre of the metal be removed, and adjust the instrument so that the image may be as sharp and distinct as possible. Then replace the central piece, and remove the outside annular one, by which means the circumference only of the speculum will be exposed; and the image now formed will be from the rays reflected from the exterior side of the metal. If the two images formed by these two portions of the metal be perfectly sharp and equally distinct, the speculum is perfect and of the true parabolic curve. If, on the contrary, the image from the outside of the metal should not be distinct and that it should be necessary to bring the little speculumnearerby the screw, the metal is not yet brought to the parabolic figure; but if, in order to procure distinctness, we be obliged to move the small speculum farther off, then the figure of the great speculum has been carried beyond the parabolic, and has assumed the hyperbolic form.
To adjust the eye-hole of Gregorian Reflectors.—If there is only one eye-glass, then the distance of the small hole should be as nearly as possible equal to its focal length. But in the compoundHuygenian eye-piece, the distance of the eye-hole may be thus found:—Multiply the difference between the focal distance of the glass next to the speculum, and the distance of the two eye-glasses, by the focal distance of the glass nearest the eye; divide the product by the sum of the focal distances of the two lenses, lessened by their distance, and the quotient will be the compound focal distance required. Thus, if the focal distance of the lens next the speculum be 3 inches, that of the lens next the eye 1 inch, and their distance 2 inches, then the compound focal distance from the eye-glass will be (3 - 2 × 1)/(3 × 1 - 2) = ½ inch.—Thediameterof the eye-hole is always equal to the quotient obtained by dividing the diameter of the great speculum by the magnifying power of the telescope. It is generally from1/25th to1/50th of an inch in diameter. It is necessary, in many cases, to obtain,from direct experiment, an accurate determination of the place and size of the eye-hole, as on this circumstance depends, in a certain degree, the accurate performance of the instrument.
To center the two specula of Gregorian Reflectors.—Extend two fine threads or wires across the aperture of the tube at right angles, so as to intersect each other, exactly in the axis of the telescope. Before the arm is finally fastened to the slider, place it in the tube, and through the eye-piece (without glasses) the intersection of the cross wires must be seen exactly in the centre of the hole of the arm. When this exactness is obtained, let the arm be firmly riveted and soldered to the slider.
To centre lenses.—The centering of lenses is of great importance, more especially for the object-glassesof achromatic instruments. The following is reckoned a good method:—Let the lens to be centered be cemented on a brass chuck, having the middle turned away so as not to touch the lens, but near the edge, which will be hid when mounted. This rim is very accurately turned flat where it is to touch the glass. When the chuck and cement is warm it is made to revolve rapidly: while in motion a lighted candle is brought before it, and its reflected image attentively watched. If this image has any motion, the lens is not flat or central; a piece of soft wood must therefore be applied to it in the manner of a turning tool, till such time as the light becomes stationary. When the whole has cooled, the edges of the lens must be turned by a diamond, or ground with emery.
For more particular details in reference to grinding and polishing specula and lenses, the reader is referred to Smith’s ‘Complete system of Optics’—Imison’s ‘School of Arts’—Huygenii Opera—Brewster’s Appendix to ‘Ferguson’s Lectures’—‘Irish Transactions,’ vol. X., or ‘Nicholson’s Journal,’ vol. XVI., Nos. 65, 66, for January and February, 1807.
A micrometer is an instrument attached to a telescope, in order to measure small spaces in the heavens, such as the spaces between two stars, and the diameters of the sun, moon and planets—and by the help of which theapparent magnitudeof all objects viewed through telescopes may be measured with great exactness.
There are various descriptions of these instruments, constructed with different substances, and in various forms, of which the following constitute the principal variety. TheWiremicrometer—theSpider’s linemicrometer—the Polymetric reticle—Dividedobject glassmicrometer—Dividedeye-glassmicrometer—Ramsden’sCatoptricmicrometer—Rochon’scrystalmicrometer—Maskelyne’sPrismaticmicrometer—Brewster’smicrometrical telescope—Sir W. Herschel’sLampmicrometer—Cavallo’sMother of Pearlmicrometer, and several others. But, instead of attempting even a general description of these instruments, I shall confine myself merely to a very brief description ofCavallo’s Micrometer, as its construction will be easily understood by the general reader, as it is one of the most simple of these instruments, and is so cheap as to be procured for a few shillings; while some of the instruments now mentioned are so expensive, as to cost nearly as much as a tolerably good telescope.37
This micrometer consists of a thin and narrow slip of mother of pearl finely divided, which is placed in the focus of the eye-glass of a telescope, just where the image of the object is formed; and it may be applied either to a reflecting or a refracting telescope, provided the eye-glass be a convex lens. It is about the 20th part of an inch broad, and of the thickness of common writing paper, divided into equal parts by parallel lines, every fifth and tenth of which is a little longer than the rest. The simplest way of fixing it is to stick it upon the diaphragm which generally stands within the tube, and in the focus of the eye-glass. When thus fixed, if you look through the eye-glass, the divisions of the micrometrical scale will appear very distinct, unless the diaphragm is not exactly in the focus of the eye-glass, in which case it must be moved to the proper place;—or, the micrometer may be placed exactly in the focus of the eye-lens by the interposition ofa circular piece of paper, card, or by means of wax. If a person should not like to see always the micrometer in the field of the telescope, then the micrometrical scale, instead of being fixed to the diaphragm, may be fitted to a circular perforated plate of brass, of wood, or even of paper, which may be occasionally placed upon the said diaphragm. One of these micrometers, in my possession, which contains 600 divisions in an inch, is fitted up in a separate eye-tube, with a glass peculiar to itself, which slides into the eye-piece of the telescope, when its own proper glass is taken out.
To ascertain the value of the divisions of this micrometer.—Direct the telescope to the sun, and observe how many divisions of the micrometer measure its diameter exactly. Then take out of the Nautical Almanack the diameter of the sun for the day on which the observation is made. Divide it by the above-mentioned number of divisions, and the quotient is the value of one division of the micrometer. Thus, suppose that 26½ divisions of the micrometer measure the diameter of the sun, and that the Nautical Almanack gives for the measure of the same diameter 31´: 22´´, or 1882´´. Divide 1882 by 26.5, and the quotient is 71´´ or 1´: 11´´, which is the value of one division of the micrometer; the double of which is the value of two divisions, and so on. The value of the divisions may likewise be ascertained by the passage of an equatorial star over a certain number of divisions in a certain time. The stars best situated for this purpose are such as the following—δ in the Whale, R. A. 37°: 3⅓´, Dec. 37´: 50´´ S; δ in Orion, R. A. 80°: 11´: 42´´, Dec. 28´: 40´´ S; υ in the Lion, R. A. 171°: 25´: 21´´, Dec. 23´: 22´´ N.; η in Virgo R. A. 182°: 10´, Dec. 33´: 27´´N. But the following is the most easy and accurate method of determining the value of the divisions:—
Mark upon a wall or other place the length ofsix inches, which may be done by making two dots or lines six inches asunder, or by fixing a six inch ruler upon a stand. Then place the telescope before it, so that the ruler or six-inch length may be at right angles with the direction of the telescope, and just 57 feet 3½ inches distant from the object-glass of the telescope; this done, look through the telescope at the ruler, or other extension of six inches, and observe how many divisions of the micrometer are equal to it, and that same number of divisions is equal to half a degree, or 30´; and this is all that is necessary for the required determination. The reason of which is, because an extension of six inches subtends an angle of 30´, at the distance of 57 feet, 3½ inches, as may be easily calculated from the rules of plane Trigonometry.