Tenure (Lat. tenere, to hold), in its most general sense, the mode of holding property.

In law it is usually confined to the manner of holding land or real property. The first grand division of tenures is into allodial tenures and feudal tenures. Of the word allodial, both the origin and the exact original meaning are uncertain. Practically it means a tenure which unites the right of the lord and the right of the tenant, or all right and title to or interest in the land. Hence, one who held land by allodial tenure had full and unencumbered possession of it, with an absolute right to use and dispose of it at his own pleasure, with no control of any one, and no responsibility to any one. An allodial holding stands in direct contrast with a feudal tenure, of which it was the essential quality that a tenant held it of a lord, and that tenant and lord had each their separate rights and interests in it and over it, or, in the language of the law, their separate estates in it. From this characteristic of allodial tenure, it is sometimes said that all the land in the United States is held by this tenure. - It seems to be generally admitted that previous to the prevalence of the feudal system the lands of European nations were held by allodial tenure, and that during the convulsions of the 9th, 10th, and 11th centuries, it became common for holders of land voluntarily to convert their allodial tenure into a feudal tenure, and so hold of some lord.

One reason, and probably the strongest, was to obtain his support and protection in return for the allegiance of the tenant; but it may be believed that another cause of this change was the general desire to profit by the opportunity which the feudal system offered of escaping from the disordered and fragmentary condition of society then prevalent. This feudal system was nowhere more fully developed or more firmly established than in Normandy. It was therefore a matter of course that when William acquired England under a claim of title, but by the power of a feudal army which he carried with him, he should establish his victorious chiefs upon the land their arms had won under that feudal system which was admirably adapted to give to the sovereign lord, at any moment, a martial array that should combine nearly all the available force of the country, and be supported by all its available resources. He divided the land in unequal portions, observing that gradation of rank and of possession which constituted a characteristic feature of the system. While ho who received a single manor became a baron and had his own court, they who received six or more were originally classed as greater barons; and to some of his principal chiefs he gave as many as 700 manors.

In this way he divided most of the valuable land of England. His immediate successors followed the same system, and before a century had elapsed the feudal system and the feudal tenures were established over nearly all England. All these tenures rested upon the fee (see Fee); but they were very various, and divided the interest in and the beneficiary use of the land, between time for a "protection" or other equivalent for a patent. Metius made a similar present and a similar application later in the same month, but said that he had manufactured such instruments two years before. It has been frequently said that Zacharias Jansen also invented the telescope more than a year later; but the evidence adduced only proves, according to Olbers, that he made telescopes which may have been imitated from those of Lippersheim; and this is the more likely as both were spectacle makers in the same city, and it is hardly possible that the public transaction with the latter could have escaped the knowledge of Jansen. The attempt was made by the states general, it is said, to retain to themselves the knowledge of this invention, the importance of which in war was at once perceived by Prince Maurice; but it is also believed that the French ambassador soon obtained from them an order for two telescopes for his own government.

It is certain that the report of the invention soon spread abroad, and the instruments found their way to London, Paris, and Venice. But by no one was the idea more eagerly welcomed, or its great importance more quickly recognized, than by Galileo, then visiting Venice. He was evidently willing, at a later day, to be thought the second inventor, guided only by an uncertain rumor; but it is said that he actually saw one of the Dutch telescopes. Returning to Padua with some lenses, he immediately began to improve upon what he had seen, if not to experiment independently under guidance of the mere report, and he soon found a better and more certain result than had been chanced upon by the original inventor. He made a leaden tube, and fitted at one extremity a double convex lens for object glass, and at the other a double concave for eye piece. This, his first telescope, magnified only three times; he then made another of more than double this power, and soon after, with a magnifying power of 30, he began to study the heavens, where his first discoveries excited more wonder than that of the "optic glass" itself. The popular curiosity was so great, as he himself tells us, that he was compelled night after night to stand by his glass to show its wonderful performances.

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; and the milky way was resolved into stars. The Galilean telescope produces an erect image. The object glass A B would form an inverted image at b a, fig. 3, but the concave eye lens N refracts the rays, which being produced backward forms an upright image at a' b. 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 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 were exposed for sale in Paris in the early part of the same year. These first telescopes are supposed to have been all made with a concave eye lens. Kepler in 1011 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 Elioe (1645). This eye lens gives a much larger field of view, but shows objects inverted.

On the other hand, the Galilean telescope had the advantage of greater distinctness and brightness than was found in the "astronomical" form. The true cause of this advantage 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 recon-noissances, 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.

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 rendered practically imperceptible. Enormously long telescopes were therefore constructed, and it was with them that the brilliant discoveries of that time were made. Huygens used telescopes of his own manufacture, and one of his object glasses, 123 ft. 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. The elder Campani, at Rome, made lenses of from 70 to 136 ft. focus, and with these Cassini discovered two of the satellites of Saturn. Cassini also used other lenses made by Borelli of 40 and 70 ft., and by Hartsoeker of not less than 250 ft. 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 bo used " with full aperture," and yet show no colors), by Sir Christopher Wren, and others. But the main reliance of the astronomer until near the close of the century was in the aerial telescope, with which, unwieldy as it was, 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 was an increase of aberrations which caused distaste for the plan, and it was not until about 1059, when Huygens 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, 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 " Huygenian eye piece" remains one of the best combinations for ordinary viewing purposes. Another eye piece, less successful, was constructed by Campani with three lenses so arranged as to show objects "without any iris or rainbow colors." - The refracting telescope remained full three quarters of a century without further material improvement.

Morin, professor of mathematics in the college de 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. 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 argued earnestly in favor of the pinnules for observing, from a want of confidence in the new method. As early as 1641 Gascoigne, an accomplished young English astronomer, had applied fixed threads to the telescope, and had also invented the wire or filar micrometer, He perished at the battle of Marston Moor, and his invention, of which no account had been published, remained forgotten until nearly 30 years after, when an opportunity for reclamation occurred upon the reinvention of the micrometer by Auzout. About the same period 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 idea, commenced a new era in observation. (See Transit Circle.) - Mersenne, in his correspondence with Descartes, had before 1639 suggested the practicability of using a concave mirror instead of the principal lens in the telescope.

In 1663 James Gregory of Edinburgh published, in his 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 transmitted to the eye. The rays having crossed twice, objects would appear in their natural position. An unsatisfactory attempt was made to construct such a telescope. Newton now took up the study. He soon found the true cause of the prismatic colors, and concluded " 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." Thus 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 [nicety] 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 produced an instrument with which he could discern the " concomitants " of Jupiter and the phases of Venus. Another one made soon after (1671), having a speculum of 1½ in. diameter and 6⅓ in. focus, was presented by him to the royal society of London, by whom it is still preserved.

In these telescopes the mirror M, fig. 4, is at the lower end of the tube, the mouth of which is directed toward the object to be observed. The rays 1 and 2 from one end of the object being reflected toward a, and the rays 3 and 4 from the other end toward b, an inverted image of the object would be formed at 1) a; but a small plane mirror M;, interposed *at an angle of 45°, diverts the image to a' b', and the eye lens O magnifies this into A B. In the same year that Newton's new telescopes were made, Cassegrain, a Frenchman, proposed still another construction. 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 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 made a mirror 6 in. in diameter and with a focal length of 62 in., 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 aerial telescope of Huygens, it proved fully a match for the refractor, except that the latter showed objects somewhat brighter.

After this period reflectors came rapidly into general use, and have ever since 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. Another important memoir upon the same subject, by the Rev. John Edwards, was published in the appendix to the "British Nautical Almanac" for 1787. (See Speculum.) - About 1766 a small telescope, only 2 ft. long, fell into the hands of a German organist residing in Bath, England. He sent to London for a larger instrument, and, finding its cost too great, undertook to make one for himself. That organist was the elder Herschel. 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 ft., as well as several of the Gregorian form of 10 ft. focus.

His discovery of the planet Uranus, in 1781, brought him to the notice of George III., by whose liberality he was enabled in 1785 to undertake the construction of the celebrated 40-foot reflector, which was pronounced finished in August, 1789; but it never accomplished any work worthy of its dimensions. In it the mirror M, fig. 5, was slightly inclined, so that the image of the object was formed at b a, near the eye lens O, which magnified it into b' a'. It is commonly said that the sixth satellite of Saturn was discovered with it; but this is a mistake, the satellite having been in reality detected with one of Herschel's 18-inch reflectors. After the lapse of 50 years, during the latter portion of which the telescope had lain unused, it was dismounted by Sir John Her-schel 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. - 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 weight of Newton's authority was sufficient for a time to repress further investigations in this direction; and it was not till 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 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. But 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.

He 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. He 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. Euler 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 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. Attempting, in 1758, to make double object glasses of short focal distance to be used with a concave eye lens, Dollond found difficulties in the management of the spherical aberration, and there-. upon the idea occurred to him of dividing this aberration by having two lenses of crown glass and including the Hint lens between them; an arrangement which accomplished the purpose in view, but did not succeed with convex eye pieces also. His son Peter resumed these experiments, and presented to the royal society of London a triple object glass of 3½ ft. focal length and 3f in. aperture, with which the telescopic image was pronounced by Short, an excellent judge, to be "distinct, bright, and free from colors." 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 were the precepts given by Klugel, in his "Dioptrics," viz.: 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 surface, 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 ⅓ to ⅔, the proportion of aperture to focal length could be materially increased without prejudice to the performance of the instrument. Not long afterward 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 the mean rays at the centre and at a given distance therefrom into one and the same point. Both lenses should be concavo-convex. With a proportion of aperture to focal length of 1/13 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 found and conquered the practical difficulty, and in 1860 arrived at complete success in the manufacture of the Gaussian object glasses. - The proper construction of eye pieces was also a matter of some consideration. Besides the Huy-genian form, 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 improving the four-glass eye pieces already in use, obtained by adding a fifth lens a combination which very satisfactorily effected both the desired objects. - Joseph Fraunhofer studied the theory of light and the laws to which it is subject in transmission through various media, and solved the difficulty of procuring disks of homogeneous flint glass. 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. He completed in 1824 the splendid telescope for the observatory at Dorpat. The object glass of this instrument, double and not triple as sometimes stated, has a clear aperture of 9.6 in., and a focal length of 170.5 in. Its optical performance is of the highest character. It gave to the stellar images a perfect sharpness of definition, which enabled it not only to resolve the closest known double stars, but also to discover as double or multiple others that had passed unchallenged before the exquisite 20-foot reflectors and the practised eye of the younger Herschel. Fraun-hofer's style of "mounting" the telescope remains to this day essentially unimproved.

Galileo's Telescope.

Fig. 3. - Galileo's Telescope.

Newton's Telescope.

Fig. 4. - Newton's Telescope.

Herschels Telescope.

Fig. 5. - Herschels Telescope.

The manufacture of optical glass has received much attention in England. In 1824 a committee was appointed by the royal society to take into consideration the theory and to experiment upon the manufacture of such glass. The chief labor devolved upon three members, G. Dollond, Faraday, and Herschel. The first results were reported to the society in 1829. The efforts of this committee were directed to the manufacture of very heavy glass, and they obtained disks of 7 in., which seemed, so far as tried, to answer all the requirements of the telescope. Dr. Ritchie also devoted much attention for several years to the same subject, and with considerable success, but was prevented by premature death from publication of any of his processes. Judging by the appearance of Ritchie's glass, Mr. Simms inferred that it had been fused in moulds and there subjected to pressure. The largest disk had 7⅜ in. diameter, and was ground for use by Simms himself. It was found to be "an excellent glass, but not altogether faultless." The idea occurred to some that the desired achroma-ticity might be obtained by separating the lenses and placing the flint at some distance down the tube in the narrowing cone of rays.

In 1828 Alexander Rogers proposed to introduce in combination with the crown lens a smaller compound lens of plate and flint glass, in which the refraction is entirely destroyed, and the outstanding dispersion left available for the desired correction of that of the outer lens. The investigation of the requisite curvatures of this compound lens was found to present no peculiar difficulty; and moreover the final perfection of the compensating action could be accomplished by proper adjustment of the relative positions of the lenses, so that less rigorous accuracy is requisite in their mechanical formation. Rogers found it probable that a telescope of 18 ft. focal length, with a crown lens of 12 in. aperture, could be made achromatic with a flint lens only 4 in. in diameter; and four years later this construction was introduced into use by Plossl at Vienna with much success. It received the name of "dialytic" or separated telescope. One of these telescopes, in the possession of Schumacher, having an aperture of 20¼ in. and focal length of 2 ft., was described by him as of extraordinary excellence of defining power.

Struve compared a dialytic telescope of 30¼ in. aperture, bearing a magnifying power of 135, with a Fraunhofer telescope of half an inch greater aperture and a power of 210, and was scarcely able to perceive any superiority in the latter. Telescopes with lenses of rock crystal and glass were advertised to be made in Paris by Cauchoix in 1831, and some few came into favorable notice; but the difficulty of obtaining the materials in proper shape and size will be a permanent obstacle to their general manufacture. It had long been observed that, even in the best telescopes, there were residual colors having their origin in the want of a perfect correlation of the colored spaces in the spectra formed by the crown and flint lens; so that if any two colors be made to unite at the same focus, as in ordinary object glasses, there would not be at the same time a complete union of the rest. This want of correlation is called the "irrationality" of the colored spaces, and its effect is called the "secondary spectrum." Dr. Blair, to overcome this effect, first made each of the lenses of his object glass independently achromatic, and in such a way that their secondary spectra corrected each other.

This he accomplished by using fluid media, two lenses of which were enclosed in combination with three of glass. Moreover, in the course of his experiments, he discovered that muriatic acid combined in proper proportions with metallic antimony gave a spectrum in which the colors had exactly the same proportions as in crown glass; and therefore by enclosing this fluid between two crown lenses, one a plano-convex and the other a meniscus, he obtained a telescope absolutely free from colors. The name "aplanatic," or without error, was given to this combination. Another fluid-lens telescope, of the dialytic form, was constructed by Barlow, who made use of the high dispersive power of sulphuret of carbon, a beautifully transparent and colorless fluid. He was able to render achromatic a combination of a crown lens 8 in. in diameter with a fluid lens of half the size. There is however a practical objection to the use of sulphuret of carbon arising from the variability of its density by variations of temperature. - Reverting to what may be called the regular construction of achro-matics, we find that the successors of Fraun-hofer at Munich, and Guinand and Cauchoix at Paris, have produced object glasses of dimensions far superior to those of the Dorpat lens.

Disks of 10, 12, and even more inches in diameter have become familiar to these master opticians, whose skill in working them keeps even pace with their manufacture; and in three Munich telescopes, two with more than 15 in. of clear aperture, one at Pulkova, another at the observatory of Harvard college, and the third at Greenwich (aperture 13 in.), have been in use for years. The two former have been the means of adding largely to the stores of astronomical knowledge; the Greenwich telescope has not been much used. - The few attempts made in the United States to manufacture optical flint glass have hitherto been but partially successful, and that with only small disks; but the American-wrought object glasses have earned for themselves a high place. Many have been made in New York by Henry Fitz, whose largest glass, 13 in. in diameter, was made for the Dudley observatory at Albany. Spencer, famous for the excellence of his microscopic objectives, made for Hamilton college a 13½ inch telescope, which is highly commended. But in exqui-siteness of workmanship and performance, the object glasses made by Alvan Clark of Cambridge, Mass., have fairly distanced all competitors, native or foreign.

Whoever will glance over the list of close double stars discovered with his 7- and 8-inch lenses (see " American Journal of Science," vols. xxv. and xxix.) will remark several stars that must have passed unnoticed under the review of Struve with his superior optical power. (See Clark, Alvan.) Mr. Dawes, one of the most skilful astronomical observers of his day, took in succession five or six large refractors from Clark (disposing of each in favor of a successor including some improvement of construction which had suggested itself), and these, scattered throughout England, attested the skill of the American optician in the special work of figuring object glasses, in which at present he and his sons are unrivalled. In 1859 Clark began the construction of a magnificent object glass of 18|-in. clear -aperture and with a focal distance of 23 ft., at that time the largest in the world. It was made from disks of Birmingham glass, which have a uniform density and freedom from veins, and, though only rudely mounted at first, quickly revealed the duplicity of the minute companion of a2 Capricorni. In January, 1862, it detected a companion to Sirius, perhaps the hitherto invisible one whose workings have been indirectly manifested in the variable movement of the larger star.

This masterpiece, prevented from reaching its original destination, was secured for the Chicago observatory. In 1870 Clark was authorized by congress to begin the construction of a telescope 24 in. in aperture for the Washington observatory; but before the work was entered upon, the proposed aperture was changed to 26 in., Mr. Newall of Gateshead, England, having had a glass constructed for him by Cooke and sons, York, of the hitherto unequalled aperture of 25 in. The disks of glass, obtained by Clark from Chance and co. of Birmingham, reached Cambridge, Mass., in December, 1871, and the grinding was begun in January, 1872. " Owing to the great size of the glasses," says Prof. Newcomb, "the first rough grinding was done by machinery, the ' grindstone' being a rapidly revolving iron wheel, over which a stream of water and sand was kept running. The glasses were thus roughly brought to the desired shape in a few days. The forms chosen were much more simple than those usually employed in large glasses, the crown glass being double convex, with an equal curvature on each face; the flint nearly plane on one side, while the other was concave, with the same curvature as the crown glass. ... In the month of June, 1872, the glass was in such good shape that only an expert could see any defect whatever.

Looking through it we could read, at the distance of some 400 ft., a microscopic photograph illegible to the naked eye. . . . Artificial double stars, one third of a second apart, were clearly separated. In hands less severely critical than those of the makers it would have passed as optically perfect. Nevertheless four months more were spent on it, and it was not till October that it was reported finished. . . . The influence of temperature on its figure was now quite perceptible. In the evening, while temperature was falling, the defect of the spherical aberration was one way, but after it became stationary the defect was slightly in the opposite direction." The telescope was mounted at Washington in 1874, and though as yet it has achieved no noteworthy discovery, the ease with which it has gone through the work which had been usually regarded as closely testing the powers of the largest telescopes shows what it is capable of. - In England, the attention of the mechanical astronomers, if we may so call them, has been of late years more especially occupied with the construction of large reflecting telescopes, and preeminent in this department was Lord Rosse, who about 1844 completed a telescope which has a clear aperture of 6 ft. and a focal length of 53 ft.

This enormous instrument has two specula, one about 3½ and the other about 4 tons in weight. At first each rested upon a system of 27 platforms most ingeniously arranged to distribute their support of this enormous weight in such a manner as to produce equal pressure in every position of the instrument. A strong pressure of the hand at the back of a speculum 4 tons in weight and nearly 6 in. thick produces flexure sufficient to distort the image of a star. At a later period 27 triangles, each with a ball at each angle, were substituted for the platforms, so that now the speculum rolls freely on 81 balls. The tube of the telescope is supported upon a massive universal joint of cast iron resting upon a pier of stonework, and it is so counterpoised by a chain suspension applied at the centre of gravity that it can be moved with great facility, a quick motion being-given by a windlass below, and a controlling slow motion in either direction by the hand of the observer above. Various micrometers have been tried with this instrument, but the common filar micrometer with coarse threads answers best; and such is the quantity of light collected by the immense reflecting surface below, that the threads in the micrometer are always distinctly visible without artificial illumination even in the darkest night.

The general processes of casting, grinding, and figuring these large specula are described in the article Spect/lum. Several other large reflectors have been constructed by Lassell, De la Rue, and Nasmyth; and the first of these transported to Malta a Newtonian telescope 4 ft. in diameter. De la Rue successfully applied his large telescopes to celestial photography, in which he has made many important improvements. - The manufacture of reflecting telescopes with glass specula received a new impulse from the discovery by Liebig of a process of coating glass with an infinitesimal film of pure metallic silver. From the first days of reflectors, as early as Newton, we find a proposition to substitute a silvered lens for the metallic mirror of his telescope, on account of the greater perfection with which glass could be wrought, and the greater durability of the polished surface. In 1740 Caleb Smith showed how, with glass mirrors silvered upon the posterior surface, the rays of different refrangi-bility, after twice passing through the glass, and thus becoming separated, might be united again by the action of a small concave lens placed not far from the focus of the mirror. The elder Herschel sometimes used glass reflectors for his smaller telescopes.

In 1822 Airy proposed a combination of two silvered lenses in the Gregorian or Cassegrainian form, and showed how, by proper mutual adjustment of the two, a perfect achromatism might be obtained. In 1838, and again in 1841, Barfuss of Weimar found that, of the various forms of reflectors, the Cassegrainian was best adapted for glass mirrors. He demonstrated that in this form both chromatic and spherical aberration may be sensibly corrected in a telescope of 20 in. focus with full 5 in. aperture, and that such a telescope would bear even a power of 600. But by Liebig's discovery a still better field has been opened. His process consists in precipitating the silver upon the glass surface from an alkaline solution prepared by addition of caustic soda to the ammonio-nitrate. After immersing the glass for about three quarters of an hour, an extremely thin and regular film is obtained, which has a slight bronzy hue by reflected light, and will transmit a deep blue light when interposed between the sun and the eye. This film is said to be harder than ordinary silver, and, by friction with soft leather and perhaps a little dry rouge, is susceptible of receiving the most brilliant polish externally, while it answers perfectly in figure to that of the glass beneath.

Foucault has also made use of a similar process (see Speculum), and succeeded in constructing telescopes of considerable dimensions. One was made by him of 13 in. aperture and only 88 in. focus, with which, under a magnifying power of 600, he could separate the components of the small companion of y Andromedae. Steinheil, investigating the relative reflecting power of a speculum coated by this new process, as compared with others and with the transmitting power of some object glasses, found that, under an angle of reflection, of 45°, the amount of brightness obtained was as follows :

Direct light ......................

100

Silverd mirror ........................

91

Quicksivered glass ....................

76

M etallic mirror , one reflection .............................

67

Herschel gives also:

Newtonian telescope ................

44

Gregorian or Cassegrainian ..................

40

Steinheil:

Object glass by Fraunhofer transmits ................

76

Object glass by Steinheil ........................

87

We are now able to substitute for the heavy and intractable speculum metal a disk of glass which is far easier to cast and anneal, and being much firmer can be made of less than half the weight of the metallic mirrors. - The helioscope, for observing the sun, is a telescope with the aperture diminished as much as possible, and usually provided with shades of stained glass to protect the eye. Still, great inconvenience is felt from the intense heating .power of the concentrated solar rays. Sir John Herschel proposed to use only the very small portion of light reflected from the first surface of glass, by constructing the large mirror of a Newtonian telescope of a double-concave, well polished lens, whose first surface only is truly figured to serve as reflector for the 2.6 per cent, of rays untransmitted and unabsorbed. The lower end of the telescope tube being left open, all the remainder of the light passes out and is dispersed. But even the small amount of reflected rays is still further reduced by the second reflection, which is made to take place at the first surface of a prism whose refracting angle should not be less than 30° or 40°, so that now the portion of light finally reaching the observer is but 1/1300 of the direct illumination, in consequence of which immense reduction a very light shade only is needed.

Porro of Paris, in constructing a telescope upon this principle, improved it by placing the prism for the second reflection at the polarizing angle for glass, whereby, upon introducing a Nicol's prism, the light may be enfeebled'as much as desired without using any shade at all. - The great requisites of a telescope stand are firmness and stability, combined with a facility of motion which will allow the instrument to be pointed with ease and certainty to any part of the heavens. Fraunhofer, whose plan is now generally followed, adopted the equatorial form, as it is called, which consists essentially of a polar axis upon which the whole instrument is moved parallel with the celestial equator, and which carries in a socket another axis at right angles to itself, upon which latter the telescope moves from or toward the pole. By the combined motions command of the whole visible hemisphere is given, and with the advantage that, the instrument being once directed to a star, the observer can follow it in its diurnal path by motion upon the polar axis alone; moreover, by application of a simple train of wheelwork this motion can be effected by machinery, and the observer is thus enabled at his leisure to contemplate or to measure the objects which appear fixed as though in an immovable sky.

In the immense English reflectors, the lower end of the tube rests upon the ground or some solid support, and even then for the needful motions of the instrument powerful appliances of machinery have been required; but in latter days mechanical engi-neershave been able so to combine and counterpoise great masses of cast-iron machinery as to effect with wonderful ease every delicate movement desired by the astronomer, and now the idea of mounting even these large telescopes equatorially is growing familiar. The application of clockwork movement to such large reflectors renders it practicable to use them for celestial photography, as well as for some extremely delicate astronomical measurements. - The application of the telescope to meridian instruments will be exemplified in the article Transit Circle; but the telescope is also universally used for differential measurements. For such observations various modifications or appliances have been from time to time suggested or practised. The filar micrometer is the most common auxiliary of the telescope, and in skilful hands is capable of astonishing accuracy. (See Micrometer.) Great use has also been made of the power of producing and comparing together double images of the objects to be measured.

These double images are produced in various ways. Savery in England in 1743, and Bou-guer in France four years later, proposed, independently of each other, to measure the diameter of the sun by using two object glasses in the same telescope and with the same eye piece. In Savery's plan the glasses were all fixed so as to give two images of the sun whose outer edges were nearly in contact; and by measuring the variable distance of these edges, he obtained the corresponding variations of the semi-diameter from perigee to apogee. Bouguer made one of his object glasses movable, and thus could measure any angle from zero to his maximum limit, which was probably somewhat greater than the sun's diameter. In 1753 John Dollond invented the divided-object-glass micrometer, which has in later years, under the technical name of heli-ometer, achieved such wonders in the hands of Bessel and his followers. In this instrument the object glass itself when finished is divided into two equal segments, each of which forms its image independently of the other. "When the semi-lenses are brought to their normal position of coincidence, the two images coincide also; but when separated, the images diverge, and the angle of divergence is measured by the amount of separation of the lenses.

Thus the apparent diameter of a planet, for instance, is obtained by separating the images until their outer edges are in exact contact, and this may be more accurately perceived than the coincidence of the edge with a fine thread placed tangent to it as in the filar micrometer. Dollond proposed moreover to gain both accuracy and convenience of use by placing a divided object glass of very long focus before the speculum of a reflecting telescope, which would give a larger scale for the measurement of a given angle than would belong to a simple telescope of the same length. Fraunhofer was at the time of his death engaged in devising a heliometer which, when afterward completed, was placed at Konigsberg. Bessel, whose " Theory of the Heliometer" is one of the most elaborate and beautiful monographs of astronomy, was able with this instrument to grapple successfully with that even now most difficult practical problem, the measurement of the parallax of a fixed star. Several attempts have also been made, as by Rochon, Maskelyne, and Boscovich, to produce the double images by refraction through prisms or pairs of prisms, either beyond the object glass or sliding within the tube, as well as by dividing the small mirror of reflecting telescopes, as Ramsden suggested for the Cas-segrainian form, and Brewster for the Newtonian. Divided-eye-lens micrometers have also been made, the best form of which is that given by Airy, who found the four-glass eye piece best adapted for this purpose, and divided the second lens, counting from the object glass.

But in all the arrangements of divided lenses an essential imperfection arises from the exhibition of color and of some diffraction in the direction at right angles to that of the line of separation, and this practical inconvenience may be seriously felt in some classes of observations. On this account, it will probably yet be found most advantageous to make use of the double-refracting property of certain crystals for the separation of images. - This account would be imperfect without a sketch of the particular form of telescope employed by the American observing parties in photographing the recent transit of Venus. What was required was a large image of the sun at the focus of the object glass, or the principal focus as it is called. The size of this image is directly proportional to the focal length, and a focal length of about 40 ft. was required to give the image the desired dimensions. It would clearly have been impossible to provide telescopes of this length for distant stations, even if at Washington, Greenwich, or Paris instruments of such dimensions could be so driven by clockwork that the tube should remain constantly directed toward the sun.

It became necessary, therefore, to make use of a heliostat, or plain mirror, so worked by a driving apparatus as to deflect the sun's rays constantly in the same horizontal direction. The construction, of a plane mirror sufficiently true for this purpose was a task which fully taxed the skill even of Alvan Clark and his sons. "The slightest deviation from exactness," as Newcomb points out, " would be fatal; for instance, if a straightedge laid upon the glass should touch at the edges, but be the 100,000th of an inch above it at the centre, the reflector would be useless." The mirrors were tested by observing objects through a telescope, first directly and then by reflection from the mirror. If they were seen with equally good definition in the two cases, it would show that there were no irregularities in the surface of the mirror; while if it were concave or convex, the focus of the telescope would seem shortened or lengthened. The first test was sustained perfectly, while the circles of convexity or concavity indicated by the changes of focus of the photographic telescope were many miles in diameter.

During the total eclipse of April, 1875, the heliostat again came into play for photographic purposes, but unsuccessfully because of unwise arrangements.