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form a conception of the importance to science first attempts to gain magnifying power and of the new invention. The phases of Venus, light, by enlarging the object glasses of telequestioned hitherto, were revealed to sight; scopes, revealed a most unexpected and forthe satellites of Jupiter and the oblong shape of midable obstacle. It was found that all obSaturn were distinctly seen; the lunar moun- jects appeared strongly tinged with prismatic tains were measured; spots were found upon colors. This obstacle remained unexplained the sun's disk; the milky way was resolved into until the time of Newton, and unconquered stars; and on every side paths were opened for more than half a century longer; and the final entering upon new and more correct views of victory over it will ever remain as one of the the constitution of the universe. In 1609, the great achievements of the human intellect. same year in which Galileo's telescopes were But if at the time insurmountable, it did not made, others found their way into England, and prove unavoidable, for it was ascertained that were soon sought after with an avidity that by making the focal distance of the object was stimulated by the report of Harriot's dis- glass very great in proportion to the diameter, coveries. This young astronomer, with zeal and the colored fringes could be reduced so as to enthusiasm no whit inferior to Galileo's, made become practically imperceptible. Enormousdrawings of the moon, discovered the satellites ly long telescopes were therefore constructed, of Jupiter, and observed the spots upon the and it was with them that the brilliant discovsun. The new “cylinders," as they were called, eries of that day were made. Huyghens used were soon in general use, and amateurs were telescopes of his own manufacture, and one of greatly delighted with the now aspects of the his object glasses, 123 feet in focal length, is heavenly bodies, especially of the moon, where still to be seen in the library of the royal sothey found luminous points "like starres” ap- ciety of London. English makers also produced pearing separated from the main illuminated telescopes of nearly equal dimensions, and portion of the new moon, and where, as one Auzout in Paris spoke of surpassing all others, expresses himself, “the whole brimme along but it does not appear whether he succeeded lookes like unto the description of coasts in the or not. The elder Campani, at Rome, made Dutch bookes of voyages.” Telescopes were lenses of from 70 to 136 feet focus, and with also exposed for sale in Paris in the early part these Cassini discovered 4 of the satellites of of the same year. It is generally supposed, in Saturn, an addition to astronomical knowledge the absence of evidence to the contrary, that which was thought worthy of commemoration these first telescopes were all, like Galileo's, by a medal. Beside the lenses of_Campani, made with a concave eye lens, a construction in some of which are still preserved in Paris, Caswhich, as has been said already, objects appear sini used also others made by Borelli of 40 and in their natural position, but with a very lim- 70 feet, and by Hartzoecker of not less than 250 ited field of view. Kepler, in 1611, suggested feet focus. These object glasses were used withthe use of a convex eye lens; but the first ac- out any tube, the lens being placed upon a mast, tual application of one was made by the capu- or, as Cassini recommended, at the angle of a chin Schyrle de Rheita, who describes it in his tower, and controlled, not without considerable work Oculus Enoch et Elio (1645). This eye difficulty, by cords leading to the observer at the lens gives a much larger field of view, but shows eye lens. The source of the inconveniences atobjects inverted. On the other hand, the Gal- tending the use of shorter lenses was generally ilean telescope possessed the advantage of supposed to lie wholly where it did really greater distinctness and brightness than was lie in part, in the imperfect collection of the found in the "astronomical” form. It was rays of light, which were at that time believed imagined by some that a sort of interference of to be homogeneous, into a simple focus. It rays took place at the crossing point, and at was distinctly understood that the rays which first sight, and while the true cause of the in- passed through a lens near its centre would not distinctness was unknown, the opinion appear. be refracted to precisely the same point with ed plausible. Even as late as 1776, the elder those which pass through it near its circumHerschel thought it worth while to disprove ference; that is, there would be what is techthis idea by an actual experiment, devised with nically called spherical aberration. This is a his characteristic ingenuity. The true cause true cause, but by no means the whole cause of of the advantage of the Galilean form is now the indistinctness of images in the telescope. known to lie in the partial compensation by Accordingly, with that belief, it was thought the negative eye piece of the aberrations caused the evil might be remedied by grinding lenses by the object glass, the result being in this case with other surfaces than spherical, and mathe difference, while in the astronomical tele- chines were devised by Descartes, by Hevelius scope it is the sum, of the aberrations of the of Dantzic, by Du Son of London (who ground two lenses. Rheita invented also the binocular deep parabolic concave lenses, with which he or double telescope, a construction which fre- asserted that telescopes might be used " with quently recurs afterward, but always as a thing full aperture," and yet show no colors), by Sir of curiosity rather than of practical utility until Christopher Wren, and others. The main reliin modern days, as the double opera glass or ance of the astronomer however until near the lorgnette, it has become serviceable in recon- close of the century was in the aërial telescope, noissances, terrestrial and celestial.-The very with which, unwieldy though it might be, many
brilliant discoveries were made.-An improve- with only the feeble optical power at their
second small concave mirror, be reflected back Frenchman, proposed still another construcby the latter, and, crossing again near the open- tion, which' bore a similar relation to the Greing in the first reflector, would be there receiv- gorian that the Galilean refractor does to Keped by a lens and thus transmitted to the eye. ler's. The large mirror was perforated, but The rays having crossed twice, objects would the rays proceeding from it were, before reachappear in their natural position. An attempt ing their focus, received upon a small convex was actually made to construct one of these mirror which sent them back with less contelescopes; but owing to want of a proper ap- vergence to form the image near the eye piece. preciation of the extreme nicety required in It was asserted that this form, which, like Gregthe figures of the mirrors, and in the relative ory's, was not immediately brought into use, adjustment, no satisfactory result was obtained. would possess several advantages over the NewIn 1671 Newton took up the study. By those tonian; but the English philosopher showed beautiful and simple experiments, the account that these advantages were rather objections, of which in his own words so highly claims our and that the difficulty of properly working the admiration, he soon found the true cause of mirrors would always be a serious obstacle to the prismatic colors which had proved such a their general acceptance. In fact, we hear litstumbling block to the progress of the instru- tle more of them until some 70 or 80 years ment, and arrived at the conclusion “ that the later, when Short, a celebrated artist of Edinperfection of telescopes was hitherto limited, burgh, revived their manufacture, and, by his not so much for want of glasses truly figur- peculiar skill in figuring and mutually adapted according to the prescriptions of optic au- ing the mirrors, “ marrying them," as he termthors, .... as because that light itself is a ed it, brought them into favor for a time. heterogeneous mixture of differently refrangi- But practical difficulties, especially in the mable rays. So that, were a glass so exactly nipulations of the large speculum, interposed figured as to collect any one sort of rays into for many years to prevent even the Newtoone point, it could not collect those also into nian construction from coming into general the same point which, having the same inci- use. It was known indeed that in order to redence upon the same medium, are apt to suffer flect all the rays accurately to the same focus, a different refraction.” Abandoning therefore the figure of the mirror should be not spherias hopeless all further attempts at improve- cal but parabolic; but no method was known ment in this direction, he was led “to take whereby this figure could be attained with cerreflectors into consideration,” since here there tainty. At length, in 1718, Hadley succeeded would be no separation of colors; but inas- in making a mirror 6 inches in diameter and much as any irregularity of figure in a concave with a focal length of 62 inches, which bore mirror would produce greater distortion in the a magnifying power of 230. This instrument image than would be the case with a lens, “a may be considered to have established the much greater curiosity would be requisite than reputation of reflectors; for on being comin figuring glasses for refraction.” The Grego- pared by Bradley and Pound with the 123-foot rian construction, mentioned above, appeared aërial telescope of Huyghens, it proved fully & to him to have such disadvantages, that he match for the refractor, except that the latter "saw it necessary to alter the design, and place showed objects somewhat brighter. But those the eye glass at the side of the tube.” Having practised observers were able to see nothing then found an alloy of copper and tin which with the Huyghenian telescope which they appeared to possess the requisite qualities for could not see also with the reflector, and somemirrors, and having also devised a “tender times the latter had the advantage. After this way of polishing proper for metal,” he at- period reflectors came rapidly into general use, tempted the construction of a reflecting tele- and have ever since, as indeed they were from scope upon the plan which has ever since borne the first, been the favorite kind of telescope in the name of Newtonian, and soon succeeded in England. Their construction was greatly famaking an instrument with which he could cilitated to practical men by the appearance in discern the “concomitants" of Jupiter and the 1777 of an elaborate memoir by Mudge, giving phases of Venus. Another one made soon af- a detailed account of his process of making and ter, having a speculum of 1} inches diameter finishing specula; a memoir for which the auand 64 inches focus, was presented by him to thor received the royal society's gold medal. the royal society of London, by whom it is still Another important memoir upon the same subpreserved. In these telescopes the mirror is ject by the Rev. John Edwards, was published placed at the lower end of the tube, whence it in the appendix to the “British Nautical Alreflects the rays forward toward a focus; but manao" for 1787.—About the year 1766 a small before reaching this point, their course is di- telescope, only two feet in length, fell into the verted by a small plane mirror inclined to the hands of a German organist residing in Engaxis at an angle of 45°, so that the image is land. His curiosity and zeal were both arousformed at the side of the tube near its mouth, ed; he sent to London for a larger instrument, and is there viewed by the eye lens, so that the and, finding its cost too great for his then observer looks in a direction at right angles limited means, undertook to make one for himto that of the object. In the same year that self. The organist was the elder Herschel. Newton's telescopes were made, Cassegrain, & With rarely equalled perseverance and mechan
ical ingenuity, he devoted all the time at his as to produce an image free from colors. command to the manufacture of reflectors. Im- Telescopes were made according to his direcproving continually upon his successive results, tions, and were said to perform well; but and with increasing means at his disposal, he the secret of their construction died with him, made many Newtonian reflectors, some even and no public account of the facts was giren as large as 20 feet, as well as a number of the until called forth by later occurrences. In 1747 Gregorian form of 10 feet focus; in the course Euler, referring to the construction of the huof all which work he acquired naturally most man eye, declared that a combination of lenses profitable experience. Astronomical observa- of different media was possible which should tion also went hand in hand with his me- give a colorless image, and investigated anachanical progress, and the pages of the “ Phil- lytically the curvatures for a lens compounded osophical Transactions” bear abundant testi- of glass and water. His result was questioned, mony, not only to his skill and success in ob- singularly enough, by the man from whom serving, but also to the great philosophical opposition might have been least expected, powers of his mind. The discovery by him of John Dollond, who, relying too implicitly upon the planet Uranus, in 1781, brought him to the Newton's dictum, was contending against his favorable notice of George III., by whose liberal own future fame. His questioning, however, patronage he was enabled in 1785 to undertake partook more of the nature of the same dicthe construction of the celebrated 40-foot re- tum itself than of argument, and he was soon flector, which under these auspices progressed led to consider the subject more attentively rapidly to completion. The instrument was by the remark of a Swedish mathematician, pronounced finished in Aug. 1789, when the that there were certainly some cases to which labors of the persevering and zealous astrono- Newton's rules did not apply. Thus shaken mer received their first reward in the discovery in his confidence, Dollond undertook experiof the 6th satellite of Saturn. The tube of this ments, at first with prisms of glass and water, great telescope was of sheet iron, nearly 40 feet and soon found that when the prisms were so in length, and more than 4 feet in diameter. combined that the rays passed through withThe speculum, weighing 2,118 pounds, bad 48 out refraction, they were tinged with the colors; inches of effective aperture, and was 34 inches next, arranging the prisms so that the rays apthick. By slightly inclining it, the rays form- peared without colors, he found them displaced ing the image were thrown to one side of the by refraction. Here, then, was in his hands tube, just beyond its mouth, and there received the grand principle which was to make a revo by the eye piece directly, thus saving the per- lution in the construction of the telescope. He centage of light ordinarily lost by the second pursued the study, and arrived at the same reretlection. The motion of this massive instru- sults by using prisms of crown and flint glass. ment was effected by means of a symmetrical From prisms to lenses the transition was easy, arrangement of masts and ladders, which form- and his triumph was finally completed, when, ed a framework or scaffolding within which having combined a convex lens of crown glass the telescope could be not only supported, but with a suitable concave of flint, he was able to directed with ease and certainty to any part of correct the colors and leave sufficient refraction the heavens. After the lapse of 50 years, dur- outstanding, to produce a telescopic image. ing the latter portion of which the telescope And now, singularly again, it was Euler's turn had lain unused, it was dismounted by Sir John to doubt. He still believed all kinds of glass Herschel at the end of 1839, and on New alike in their optical properties, and that it was Year's eve his family assembled within the only some happy combination of curvatures at tube and sang its requiem. It now rests hori- which Dollond had arrived. But his doubts zontally upon three stone pillars, a monument soon gave way before experience, and the masto the memory of its constructor.-It would terly powers of his analysis were brought to seem that the improvement of the refracting bear successfully upon the problem of the comtelescope after the middle of the 17th century pound object glasses. The subject attracted was long retarded in consequence of the opinion, universal attention, and mathematicians everyquoted above, of one whose views were al- where contributed toward perfecting by theory ways to be received with deference. Newton the requisite conditions of curvature of the evidently conceived that the prismatic rays of lenses. The new telescopes were appropriately light, once separated, could not be recomposed called achromatic, or free from color, and henceinto white light except by the same refraction forth the “dispersive power” of any medium, that had separated them, and that therefore by virtue of which the differently colored rays the removal of these colors from a telescopic are differently refracted—that is, are dispersed image was impossible. The opinion was seem- from each other—was recognized as indepeningly self-evident, and yet was incorrect. The dent of the “refractive power,” by virtue of weight of Newton's authority, however, was which the whole pencil is diverted from its sufficient for a time to repress further investi- original source. Side by side with the theogations in this direction; and it was not until rists was Dollond with his practical skill. At1729 that an Englishman named Hall, guided, tempting, in 1758, to make double object glasses it is said, by a study of the mechanism of the of short focal distance to be used with a con. eye, was led to a plan of combining lenses so cave eye lens, he found difficulties in the man.
agement of the spherical aberration, whereupon the mean rays at the centre and at a given disthe idea occurred to him of dividing this aber- tance therefrom into one and the same point. ration by having two lenses of crown glass and Both lenses should be concavo-convex in form, including the flint lens between them, an ar- and with a proportion of aperture to focal rangement which accomplished the purpose in length of he obtained an almost perfect view, but did not succeed with convex eye union of rays. The unusually deep curvatures pieces also. Afterward, his son Peter, whose of the lenses seem to have occasioned some own skill combined with his father's inventive scruples on the part of opticians, and this congenius to render the name of Dollond insepa- struction remained almost forgotten for 40 rably associated with the history of the telescope, years, until Steinheil, with characteristic boldresumed these experiments, and as a result pre- ness, has recently taken up the study, found and sented to the royal society of London a triple conquered the practical difficulty, and has arobject glass of 3 feet focal length and 3inches rived (in 1860) at complete success in the manaperture, with which the telescopic image was ufacture of the Gaussian object glasses. --The pronounced by Short, an excellent judge, to be proper construction of eye pieces was also a
distinct, bright, and free from colors." The matter of some consideration. Beside the Huyproper combination of three lenses into a sys- ghenian form already mentioned, and which is tem in which any the least departure from har- only applicable for viewing objects, Ramsden, monious action will offend the practised eye, in 1783, introduced another, which is still used if not vitiate the whole action, was of course in micrometer observations. It consists of two recognized as a matter requiring the utmost plano-convex lenses, of equal focus, with their delicacy. A beautiful suggestion was made by convex surfaces toward each other, and sepWollaston of a means of testing and correcting arated by a distance of two thirds of the comthe concentric adjustment of lenses. By re- mon focal length. By this arrangement, to moving the eye glass of a telescope and view- which he was guided by a remark of Newton, ing any bright object, as a lighted candle, the essential condition of a “flat field” is through the object glass, there may be ob- gained, and the aberrations, chromatic and served at the same time with the refracted spherical, are so much reduced as to be pracimage a series of fainter images formed by the tically insensible. For terrestrial observations, second reflections from the different surfaces. the elder Dollond sought to reduce aberrations It is evident, then, that if the glasses be truly and enlarge the field of view, by increasing centred, these images will all be in the same the number of lenses, and, after having first straight line; or if there be any error of posi- improved the 4-glass eye pieces already in use, tion of either lens, it will be decidedly mani- obtained, by adding yet a 5th lens, à combifested, and by proper adjusting screws may nation which very satisfactorily effected both be corrected accordingly.--Among the many the desired objects. Among the distinguished mathematical solutions of the new problem names connected with the history of the teleof the object glasses, the precepts given by scope, some, like those of Euler and Gauss, Klügel, in his " Dioptrics," commended them- brought lustre to it; others, like Dollond's, reselves to the general apprehension, and served ceived lustre from it. The name of Joseph Fraunas a basis for subsequent fruitful investigations. hofer did both. Devoting the labors of a life (see These precepts were: 1, that the radii of cur- FRAUNHOFER) all too short for science to the imvature of the first, or crown lens, should be provement of the achromatic telescope, he studsuch that the angles of the incident ray with ied the theory of light and the laws to which it the normal would be equal at both surfaces, was subject in transmission through various which would give for crown glass a ratio of media, and, by aid of his own ingeniously nearly 1 to 3; 2, the radius of the third sur- devised apparatus, gained perfect familiarity face, the first of the flint lens, should be such with all its modes of action. Then, fully apthat the rays of mean refrangibility passing preciating the obstacles to practical application through both the centre and edge of the lens of this knowledge, lying in the difficulty of would unite as nearly as possible in the same procuring disks of homogeneous flint glass, his part of the axis, so that the spherical aberration inechanical ingenuity contended successfully would be sensibly destroyed; and 3, having against this difficulty. The process by which determined the outstanding dispersion for the his glass was manufactured is kept a secret, but red and violet rays, the fourth surface should it is generally understood that the disks thembe made such as to unite these rays as nearly selves are obtained by selecting and melting as possible in the same point with the rest. together the most faultless specimens from Early in 1816 Bohnenberger, commenting upon larger masses of the best glass, whose constituthese precepts, showed that, by changing the ent parts however are not known. Having now ratio of the first two surfaces from j to ž, the the glass, he well knew how to combine curvaproportion of aperture to focal length could be tures to suit its peculiar properties, and the rematerially increased without prejudice to the sults are to be found all over Europe. His life’s performance of the instrument. Not long after, labors were fitly crowned by the completion, Gauss remarked that it was possible, theoreti- in 1824, of the splendid telescope for the obsercally, to construct an object glass which would vatory at Dorpat, a masterpiece which excited unite all the rays of any two colors as well as no less wonder than admiration. The object