form a conception of the importance to science of the new invention. The phases of Venus, questioned hitherto, were revealed to sight; the satellites of Jupiter and the oblong shape of Saturn were distinctly seen; the lunar mountains were measured; spots were found upon the sun's disk; the milky way was resolved into stars; and on every side paths were opened for entering upon new and more correct views of the constitution of the universe. In 1609, the same year in which Galileo's telescopes were made, others found their way into England, and were soon sought after with an avidity that was stimulated by the report of Harriot's discoveries. This young astronomer, with zeal and enthusiasm no whit inferior to Galileo's, made drawings of the moon, discovered the satellites of Jupiter, and observed the spots upon the sun. The new "cylinders," as they were called, were soon in general use, and amateurs were greatly delighted with the new aspects of the heavenly bodies, especially of the moon, where they found luminous points "like starres" appearing separated from the main illuminated portion of the new moon, and where, as one expresses himself, "the whole brimme along lookes like unto the description of coasts in the Dutch bookes of voyages." Telescopes were also exposed for sale in Paris in the early part of the same year. It is generally supposed, in the absence of evidence to the contrary, that these first telescopes were all, like Galileo's, made with a concave eye lens, a construction in which, as has been said already, objects appear in their natural position, but with a very limited field of view. Kepler, in 1611, suggested the use of a convex eye lens; but the first actual application of one was made by the capuchin Schyrle de Rheita, who describes it in his work Oculus Enoch et Elia (1645). This eye lens gives a much larger field of view, but shows objects inverted. On the other hand, the Galilean telescope possessed the advantage of greater distinctness and brightness than was found in the "astronomical" form. It was imagined by some that a sort of interference of rays took place at the crossing point, and at first sight, and while the true cause of the indistinctness was unknown, the opinion appeared plausible. Even as late as 1776, the elder Herschel thought it worth while to disprove this idea by an actual experiment, devised with his characteristic ingenuity. The true cause of the advantage of the Galilean form is now known to lie in the partial compensation by the negative eye piece of the aberrations caused by the object glass, the result being in this case the difference, while in the astronomical telescope it is the sum, of the aberrations of the two lenses. Rheita invented also the binocular or double telescope, a construction which frequently recurs afterward, but always as a thing of curiosity rather than of practical utility until in modern days, as the double opera glass or lorgnette, it has become serviceable in reconnoissances, terrestrial and celestial. The very first attempts to gain magnifying power and light, by enlarging the object glasses of telescopes, revealed a most unexpected and formidable obstacle. It was found that all objects appeared strongly tinged with prismatic colors. This obstacle remained unexplained until the time of Newton, and unconquered more than half a century longer; and the final victory over it will ever remain as one of the great achievements of the human intellect. But if at the time insurmountable, it did not prove unavoidable, for it was ascertained that by making the focal distance of the object glass very great in proportion to the diameter, the colored fringes could be reduced so as to become practically imperceptible. Enormously long telescopes were therefore constructed, and it was with them that the brilliant discoveries of that day were made. Huyghens used telescopes of his own manufacture, and one of his object glasses, 123 feet in focal length, is still to be seen in the library of the royal society of London. English makers also produced telescopes of nearly equal dimensions, and Auzout in Paris spoke of surpassing all others, but it does not appear whether he succeeded or not. The elder Campani, at Rome, made lenses of from 70 to 136 feet focus, and with these Cassini discovered 4 of the satellites of Saturn, an addition to astronomical knowledge which was thought worthy of commemoration by a medal. Beside the lenses of Campani, some of which are still preserved in Paris, Cassini used also others made by Borelli of 40 and 70 feet, and by Hartzoecker of not less than 250 feet focus. These object glasses were used without any tube, the lens being placed upon a mast, or, as Cassini recommended, at the angle of a tower, and controlled, not without considerable difficulty, by cords leading to the observer at the eye lens. The source of the inconveniences attending the use of shorter lenses was generally supposed to lie wholly where it did really lie in part, in the imperfect collection of the rays of light, which were at that time believed to be homogeneous, into a simple focus. It was distinctly understood that the rays which passed through a lens near its centre would not be refracted to precisely the same point with those which pass through it near its circumference; that is, there would be what is technically called spherical aberration. This is a true cause, but by no means the whole cause of the indistinctness of images in the telescope. Accordingly, with that belief, it was thought the evil might be remedied by grinding lenses with other surfaces than spherical, and machines were devised by Descartes, by Hevelius of Dantzic, by Du Son of London (who ground deep parabolic concave lenses, with which he asserted that telescopes might be used" with full aperture," and yet show no colors), by Sir Christopher Wren, and others. The main reliance of the astronomer however until near the close of the century was in the aërial telescope, with which, unwieldy though it might be, many brilliant discoveries were made.-An improvement, of more importance than that of the figuring of lenses, consisted in the modification of the eye piece. By the introduction of more than one convex lens, Rheita had reinverted the image; but this was all the gain that either he or Kepler, who also proposed the same thing, seems to have expected. In fact, there resulted an increase of aberrations which begat distaste of the plan, and it was not until about 1659, when Huyghens invented the combination which still bears his name, that much advantage was gained by multiplying lenses. This eye piece is composed of two convex lenses, whose focal lengths are as 3 to 1, and which are separated from each other by an interval equal to half the sum of these focal lengths; the place of the telescopic image being between the lenses. This arrangement was found to have a remarkable advantage in point of distinctness over the single eye glass, by reason of the apportionment of spherical aberrations between the lenses, and the consequent less amount of injurious effect in the result, while no addition whatever was made to the color of the images formed by the object glass. To this day the "Huyghenian eye piece" remains one of the best combinations for ordinary viewing purposes. Another eye piece, less successful, however, was constructed by Campani with 3 lenses so arranged as to show objects "without any iris or rainbow colors."-In the form to which the refracting telescope had now been brought, it remained full three quarters of a century without further material improvement. But during this interval, by the application of the telescope to instruments of observation, astronomy was continually gaining in the accuracy and extent of knowledge respecting the positions and movements of the heavenly bodies. Morin, professor of mathematics in the college of France, first in 1634 attached a telescope to the moving index of a graduated arc, in order, as he says, "to measure the fixed stars quickly and accurately." He was also the first to gain sight of stars in the daytime. Having, one morning in the early twilight, directed a 14 foot telescope attached to the alidade of a planisphere upon Arcturus, he followed the star until after the sun had risen; and finding that it still continued visible, his joy was so demonstrative that his instrument was displaced and the star lost. He afterward succeeded in following Venus and other planets, as well as several fixed stars, into the daylight. But it was only after the introduction of fixed threads into the field of the telescope, that it became a really useful auxiliary to instruments of measurement. At the present day it seems at first strange that astronomers should have preferred the simple "sights" or "pinnules," with which they had always been accustomed to observe, to the far more accurate perception furnished us by the telescope; and yet they, without any means of designating the centre of the field of view, and with only the feeble optical power at their command, were right in their preference. Even as late as 1673, Hevelius, who had spent a long life in the construction and use of large instruments, as well as telescopes, argued earnestly in favor of the pinnules for observing-not, as stated by Whewell, because it would make all the old observations of no value, but rather from a want of confidence in the new method, which he said had not yet proved itself superior, especially in the case of comets. A controversy upon this question arose between him and Hooke, in consequence of which Halley was requested to visit Dantzic and make actual comparison of the two methods. He did so, when it appeared that Hevelius had acquired such skill in the use of his favorite method, that the difference of their observations seldom exceeded a few seconds, and in no case amounted to so much as a minute! As early as 1641, Gascoigne, an accomplished young English astronomer, had applied fixed threads to the telescope. In a letter to Crabtree he says that he has fitted his sextant "by the help of the cane (tube), two glasses in it, and a thread, so as to be a pleasant instrument." The object he had proposed to himself was the improvement of the lunar theory, and he "doubted not in time to be able to make observations to seconds." He had also at this time invented the wire or filar micrometer, in the details of which it is pleasant to find not merely mechanical ingenuity, but also a true appreciation of the astronomical requirements of such an apparatus; and there is no doubt that, had the inventor lived to make a more extended application of his contrivance, the results would have been of high value. Gascoigne perished untimely at the battle of Marston Moor, and his invention, of which no account had been published, and which does not seem to have been sufficiently appreciated at the time, remained forgotten until nearly 30 years after, when an opportunity for reclamation occurred upon the reinvention of the micrometer by Auzout. It was about the same period that Roemer gave to the telescope one of its most important applications, by attaching to it an axis at right angles to its length, and placing it so as to revolve in the plane of the meridian; and shortly afterward Picard in Paris, and Flamsteed at Greenwich, following up this capital idea, commenced a new era in observation. For a more detailed account of this application, see TRANSIT CIRCLE.-Mersenne, in his correspondence with Descartes, had before the year 1639 suggested the practicability of using a concave mirror instead of the principal lens in the telescope; but the idea appears to have been unfavorably received by the latter. In 1663 James Gregory, of Edinburgh, published, in his work entitled Optica Promota, the plan of a reflecting telescope, consisting of a concave mirror, perforated in the centre, by which the rays were to be converged to a focus before it, and after crossing would be received upon a second small concave mirror, be reflected back by the latter, and, crossing again near the opening in the first reflector, would be there received by a lens and thus transmitted to the eye. The rays having crossed twice, objects would appear in their natural position. An attempt was actually made to construct one of these telescopes; but owing to want of a proper appreciation of the extreme nicety required in the figures of the mirrors, and in the relative adjustment, no satisfactory result was obtained. In 1671 Newton took up the study. By those beautiful and simple experiments, the account of which in his own words so highly claims our admiration, he soon found the true cause of the prismatic colors which had proved such a stumbling block to the progress of the instrument, and arrived at the conclusion "that the perfection of telescopes was hitherto limited, not so much for want of glasses truly figured according to the prescriptions of optic authors, ... as because that light itself is a heterogeneous mixture of differently refrangible rays. So that, were a glass so exactly figured as to collect any one sort of rays into one point, it could not collect those also into the same point which, having the same incidence upon the same medium, are apt to suffer a different refraction." Abandoning therefore as hopeless all further attempts at improvement in this direction, he was led "to take reflectors into consideration," since here there would be no separation of colors; but inasmuch as any irregularity of figure in a concave mirror would produce greater distortion in the image than would be the case with a lens, "a much greater curiosity would be requisite than in figuring glasses for refraction." The Gregorian construction, mentioned above, appeared to him to have such disadvantages, that he "saw it necessary to alter the design, and place the eye glass at the side of the tube." Having then found an alloy of copper and tin which appeared to possess the requisite qualities for mirrors, and having also devised a “tender way of polishing proper for metal," he attempted the construction of a reflecting telescope upon the plan which has ever since borne the name of Newtonian, and soon succeeded in making an instrument with which he could discern the "concomitants" of Jupiter and the phases of Venus. Another one made soon after, having a speculum of 11 inches diameter and 6 inches focus, was presented by him to the royal society of London, by whom it is still preserved. In these telescopes the mirror is placed at the lower end of the tube, whence it reflects the rays forward toward a focus; but before reaching this point, their course is diverted by a small plane mirror inclined to the axis at an angle of 45°, so that the image is formed at the side of the tube near its mouth, and is there viewed by the eye lens, so that the observer looks in a direction at right angles to that of the object. In the same year that Newton's telescopes were made, Cassegrain, a Frenchman, proposed still another construction, which bore a similar relation to the Gregorian that the Galilean refractor does to Kepler's. The large mirror was perforated, but the rays proceeding from it were, before reaching their focus, received upon a small convex mirror which sent them back with less convergence to form the image near the eye piece. It was asserted that this form, which, like Gregory's, was not immediately brought into use, would possess several advantages over the Newtonian; but the English philosopher showed that these advantages were rather objections, and that the difficulty of properly working the mirrors would always be a serious obstacle to their general acceptance. In fact, we hear little more of them until some 70 or 80 years later, when Short, a celebrated artist of Edinburgh, revived their manufacture, and, by his peculiar skill in figuring and mutually adapting the mirrors, "marrying them," as he termed it, brought them into favor for a time. But practical difficulties, especially in the manipulations of the large speculum, interposed for many years to prevent even the Newtonian construction from coming into general use. It was known indeed that in order to reflect all the rays accurately to the same focus, the figure of the mirror should be not spherical but parabolic; but no method was known whereby this figure could be attained with certainty. At length, in 1718, Hadley succeeded in making a mirror 6 inches in diameter and with a focal length of 62 inches, which bore a magnifying power of 230. This instrument may be considered to have established the reputation of reflectors; for on being compared by Bradley and Pound with the 123-foot aërial telescope of Huyghens, it proved fully a match for the refractor, except that the latter showed objects somewhat brighter. But those practised observers were able to see nothing with the Huyghenian telescope which they could not see also with the reflector, and sometimes the latter had the advantage. After this period reflectors came rapidly into general use, and have ever since, as indeed they were from the first, been the favorite kind of telescope in England. Their construction was greatly facilitated to practical men by the appearance in 1777 of an elaborate memoir by Mudge, giving a detailed account of his process of making and finishing specula; a memoir for which the author received the royal society's gold medal. Another important memoir upon the same subject by the Rev. John Edwards, was published in the appendix to the "British Nautical Almanac" for 1787.-About the year 1766 a small telescope, only two feet in length, fell into the hands of a German organist residing in England. His curiosity and zeal were both aroused; he sent to London for a larger instrument, and, finding its cost too great for his then limited means, undertook to make one for himself. The organist was the elder Herschel. With rarely equalled perseverance and mechan TELESCOPE ical ingenuity, he devoted all the time at his command to the manufacture of reflectors. Improving continually upon his successive results, and with increasing means at his disposal, he made many Newtonian reflectors, some even as large as 20 feet, as well as a number of the Gregorian form of 10 feet focus; in the course of all which work he acquired naturally most profitable experience. Astronomical observation also went hand in hand with his mechanical progress, and the pages of the "Philosophical Transactions" bear abundant testimony, not only to his skill and success in observing, but also to the great philosophical powers of his mind. The discovery by him of the planet Uranus, in 1781, brought him to the favorable notice of George III., by whose liberal patronage he was enabled in 1785 to undertake the construction of the celebrated 40-foot reflector, which under these auspices progressed rapidly to completion. The instrument was pronounced finished in Aug. 1789, when the labors of the persevering and zealous astronomer received their first reward in the discovery of the 6th satellite of Saturn. The tube of this great telescope was of sheet iron, nearly 40 feet in length, and more than 4 feet in diameter. The speculum, weighing 2,118 pounds, had 48 inches of effective aperture, and was 34 inches thick. By slightly inclining it, the rays forming the image were thrown to one side of the tube, just beyond its mouth, and there received by the eye piece directly, thus saving the percentage of light ordinarily lost by the second reflection. The motion of this massive instrument was effected by means of a symmetrical arrangement of masts and ladders, which formed a framework or scaffolding within which the telescope could be not only supported, but directed with ease and certainty to any part of the heavens. After the lapse of 50 years, during the latter portion of which the telescope had lain unused, it was dismounted by Sir John Herschel at the end of 1839, and on New Year's eve his family assembled within the tube and sang its requiem. It now rests horizontally upon three stone pillars, a monument to the memory of its constructor.-It would seem that the improvement of the refracting telescope after the middle of the 17th century was long retarded in consequence of the opinion, quoted above, of one whose views were always to be received with deference. Newton evidently conceived that the prismatic rays of light, once separated, could not be recomposed into white light except by the same refraction that had separated them, and that therefore the removal of these colors from a telescopic image was impossible. The opinion was seemingly self-evident, and yet was incorrect. The weight of Newton's authority, however, was sufficient for a time to repress further investigations in this direction; and it was not until 1729 that an Englishman named Hall, guided, it is said, by a study of the mechanism of the eye, was led to a plan of combining lenses so as to produce an image free from colors. Telescopes were made according to his directions, and were said to perform well; but the secret of their construction died with him, and no public account of the facts was given until called forth by later occurrences. In 1747 Euler, referring to the construction of the human eye; declared that a combination of lenses of different media was possible which should give a colorless image, and investigated analytically the curvatures for a lens compounded of glass and water. His result was questioned, singularly enough, by the man from whom opposition might have been least expected, John Dollond, who, relying too implicitly upon Newton's dictum, was contending against his own future fame. His questioning, however, partook more of the nature of the same dictum itself than of argument, and he was soon led to consider the subject more attentively by the remark of a Swedish mathematician, that there were certainly some cases to which Newton's rules did not apply. Thus shaken in his confidence, Dollond undertook experiments, at first with prisms of glass and water, and soon found that when the prisms were so combined that the rays passed through without refraction, they were tinged with the colors; next, arranging the prisms so that the rays appeared without colors, he found them displaced by refraction. Here, then, was in his hands the grand principle which was to make a revolution in the construction of the telescope. He pursued the study, and arrived at the same results by using prisms of crown and flint glass. From prisms to lenses the transition was easy, and his triumph was finally completed, when, having combined a convex lens of crown glass with a suitable concave of flint, he was able to correct the colors and leave sufficient refraction outstanding to produce a telescopic image. And now, singularly again, it was Euler's turn to doubt. He still believed all kinds of glass alike in their optical properties, and that it was only some happy combination of curvatures at which Dollond had arrived. But his doubts soon gave way before experience, and the masterly powers of his analysis were brought to bear successfully upon the problem of the compound object glasses. The subject attracted universal attention, and mathematicians everywhere contributed toward perfecting by theory the requisite conditions of curvature of the lenses. The new telescopes were appropriately called achromatic, or free from color, and henceforth the "dispersive power" of any medium, by virtue of which the differently colored rays are differently refracted-that is, are dispersed from each other-was recognized as independent of the "refractive power," by virtue of which the whole pencil is diverted from its original source. Side by side with the theo rists was Dollond with his practical skill. Attempting, in 1758, to make double object glasses of short focal distance to be used with a con cave eye lens, he found difficulties in the man agement of the spherical aberration, whereupon the idea occurred to him of dividing this aberration by having two lenses of crown glass and including the flint lens between them, an arrangement which accomplished the purpose in view, but did not succeed with convex eye pieces also. Afterward, his son Peter, whose own skill combined with his father's inventive genius to render the name of Dollond inseparably associated with the history of the telescope, resumed these experiments, and as a result presented to the royal society of London a triple object glass of 31 feet focal length and 34 inches aperture, with which the telescopic image was pronounced by Short, an excellent judge, to be "distinct, bright, and free from colors." The proper combination of three lenses into a system in which any the least departure from harmonious action will offend the practised eye, if not vitiate the whole action, was of course recognized as a matter requiring the utmost delicacy. A beautiful suggestion was made by Wollaston of a means of testing and correcting the concentric adjustment of lenses. By removing the eye glass of a telescope and viewing any bright object, as a lighted candle, through the object glass, there may be observed at the same time with the refracted image a series of fainter images formed by the second reflections from the different surfaces. It is evident, then, that if the glasses be truly centred, these images will all be in the same straight line; or if there be any error of position of either lens, it will be decidedly manifested, and by proper adjusting screws may be corrected accordingly.-Among the many mathematical solutions of the new problem of the object glasses, the precepts given by Klügel, in his "Dioptrics," commended themselves to the general apprehension, and served as a basis for subsequent fruitful investigations. These precepts were: 1, that the radii of curvature of the first, or crown lens, should be such that the angles of the incident ray with the normal would be equal at both surfaces, which would give for crown glass a ratio of nearly 1 to 3; 2, the radius of the third sur face, the first of the flint lens, should be such that the rays of mean refrangibility passing through both the centre and edge of the lens would unite as nearly as possible in the same part of the axis, so that the spherical aberration would be sensibly destroyed; and 3, having determined the outstanding dispersion for the red and violet rays, the fourth surface should be made such as to unite these rays as nearly as possible in the same point with the rest. Early in 1816 Bohnenberger, commenting upon these precepts, showed that, by changing the ratio of the first two surfaces from to, the proportion of aperture to focal length could be materially increased without prejudice to the performance of the instrument. Not long after, Gauss remarked that it was possible, theoretically, to construct an object glass which would unite all the rays of any two colors as well as VOL. XV.-23 the mean rays at the centre and at a given distance therefrom into one and the same point. Both lenses should be concavo-convex in form, and with a proportion of aperture to focal length of he obtained an almost perfect union of rays. The unusually deep curvatures of the lenses seem to have occasioned some scruples on the part of opticians, and this construction remained almost forgotten for 40 years, until Steinheil, with characteristic boldness, has recently taken up the study, found and conquered the practical difficulty, and has arrived (in 1860) at complete success in the manufacture of the Gaussian object glasses.-The proper construction of eye pieces was also a matter of some consideration. Beside the Huyghenian form already mentioned, and which is only applicable for viewing objects, Ramsden, in 1783, introduced another, which is still used in micrometer observations. It consists of two plano-convex lenses, of equal focus, with their convex surfaces toward each other, and separated by a distance of two thirds of the common focal length. By this arrangement, to which he was guided by a remark of Newton, the essential condition of a "flat field" is gained, and the aberrations, chromatic and spherical, are so much reduced as to be practically insensible. For terrestrial observations, the elder Dollond sought to reduce aberrations and enlarge the field of view, by increasing the number of lenses, and, after having first improved the 4-glass eye pieces already in use, obtained, by adding yet a 5th lens, a combination which very satisfactorily effected both the desired objects.-Among the distinguished names connected with the history of the telescope, some, like those of Euler and Gauss, brought lustre to it; others, like Dollond's, received lustre from it. The name of Joseph Fraunhofer did both. Devoting the labors of a life (see FRAUNHOFER) all too short for science to the improvement of the achromatic telescope, he studied the theory of light and the laws to which it was subject in transmission through various media, and, by aid of his own ingeniously devised apparatus, gained perfect familiarity with all its modes of action. Then, fully appreciating the obstacles to practical application of this knowledge, lying in the difficulty of procuring disks of homogeneous flint glass, his mechanical ingenuity contended successfully against this difficulty. The process by which his glass was manufactured is kept a secret, but it is generally understood that the disks themselves are obtained by selecting and melting together the most faultless specimens from larger masses of the best glass, whose constituent parts however are not known. Having now the glass, he well knew how to combine curvatures to suit its peculiar properties, and the results are to be found all over Europe. His life's labors were fitly crowned by the completion, in 1824, of the splendid telescope for the observatory at Dorpat, a masterpiece which excited no less wonder than admiration. The object |