country, in consequence, enjoyed an exclusive privilege in the construction of instruments of this description. At length Guinand, a humble mechanic of Brenetz, a small village in the Canton of Neufchatel, Switzerland, having turned his attention to the manufacture of flint-glass towards the close of the eighteenth century, succeeded, after a long course of persevering efforts, in producing masses of that substance perfectly free of striæ, and therefore adapted for the construction of object-glasses of telescopes. Frauenhofer, the Bavarian optician, having been made acquainted with the wonderful success of Guinand, resolved to take advantage of the circumstance, and with a view to this object he induced the Swiss artizan to remove to Munich, in the year 1805. This eminent individual, who was no less remarkable for sagacity in philosophical enquiries than for skill as an artist, possessed peculiar qualifications for profiting by the instructions of Guinand, and he soon succeeded in attaining unexampled excellence in the construction of achromatic objectglasses. Telescopes were now executed by Frauenhofer, the apertures of which far exceeded anything hitherto known since the discovery of achromatism. It was with one of these instruments, having an aperture of 9.9 inches, and a focal length of 13 feet, that M. Struve made the series of micrometrical measurements of double stars at Dorpat, between the years 1824 and 1837, which have rendered the name of that astronomer so deservedly famous. With another of them, having an aperture of 12 inches, and a focal length of 18 feet, M. Lamont, of Munich, made those interesting observations of the satellites of Uranus, to which allusion has been made in one of the preceding chapters. Frauenhofer was contemplating the execution of object-glasses of still greater magnitude than either of those above-mentioned, when his brilliant career was unfortunately brought to a close by a premature death. His successors, MM. Merz and Mahler, have succeeded in effectually realising his views. Among the chef-d'œuvres of these artists may be cited the famous refractors of Pulkowa and Cambridge, U.S., to both of which instruments allusion has already been made in the foregoing part of this work. Guinand remained at Munich from 1805 till 1814, in which year he finally returned to his native country. A few years afterwards he was visited by Lerebours, an eminent French optician, who purchased of him all the flint-glass in his possession. Subsequently he also supplied Canchoix, another artist of Paris, with portions of the same valuable substance. The French opticians skilfully worked the material into object-glasses, and in this manner refracting telescopes came to be constructed in France, rivalling the most finished productions of the Munich artists. England, which continued long to be the exclusive seat of the manufacture of achromatic telescopes, had the mortification of finally seeing both Germany and France completely outstrip her in this branch of practical optics. This result she owed to the short-sighted policy of the Government, which had placed an exorbitant duty on the manufacture of flintglass. The removal of this pernicious impost a few years since, has given a new impulse to the art, and already results have been achieved which seemed to indicate that before a long period shall have elapsed, refracting telescopes will be executed in this country, rivalling the most Joseph Frauenhofer was born in the year 1787, at Straubing, in Bavaria. Besides having attained unrivalled eminence in his professional vocation, he achieved many important discoveries in physical optics. He died in 1826, while yet in the prime of life; but he left behind him an immortal reputation. powerful instruments of the same kind which have emanated from the workshops on the Continent *. While the refracting telescope has been rendered vastly more efficient in modern times, the reflecting telescope has received a corresponding degree of improvement. During the present century several very large telescopes were constructed upon this principle by the late Mr. Ramage, of Aberdeen, an individual of considerable originality, who possessed a remarkable aptitude for mechanical inventions. The perfection of these instruments does not seem, however, to have been commensurate with their magnitude, for they have been employed only to a very limited extent in astronomical observations. In more recent times, Mr. Lassell, of Liverpool, has especially distinguished himself by his skill in the construction of reflecting telescopes. With a Newtonian reflector of 2-feet aperture, which he executed with his own hands, he has discovered the single satellite of Neptune, and the eighth satellite of Saturn; while, at the same time, he has been enabled by its use, to make several very interesting physical observations of the planetary bodies. In the construction of reflecting telescopes the Earl of Rosse has attained a degree of excellence which far surpasses all previous efforts of the kind. It is impossible here to give any account of the multitude of admirable contrivances by means of which that distinguished nobleman has succeeded in bringing his telescopes to so high a state of perfection. It must suffice to state, that in 1840, he completed a Newtonian reflector of 3 feet aperture, which he subsequently employed in astronomical observations. Phenomena of a highly-interesting nature were soon disclosed by the use of this powerful instrument. Several nebulæ, hitherto observed as such in the most powerful telescopes, were resolved into clusters of stars, while others exhibited forms totally different from those which had been hitherto assigned to them. In 1845 Lord Rosse gave to the world a still more striking proof of his practical talents, and his devotion to astronomical science, by the construction of a reflecting telescope of 6-feet aperture, and 54-feet focal length! This magnificent instrument, by far the most powerful which the genius of man has hitherto executed for the purpose of exploring the grand phenomena of the heavens has already, in the hands of its noble owner, done valuable service to astronomy, by the light which it has thrown upon the structure of the nebular part of the universet. The brilliant success which has attended Lord Rosse's efforts to construct reflecting telescopes, has suggested to several eminent scientific men of the present day the expediency of transporting a powerful instrument of this description to the southern hemisphere, for the pur At the meeting of the British Association, held in the month of July of the present year (1851), at Ipswich, the Astronomer Royal, in his opening address as President of The Association, stated that Mr. Simms had completed the flint-glass for an achromatic object-glass of 13 inches in diameter, and that he was engaged in executing one of 16 inches. He also mentioned that Mr. Ross was attempting to make an object-glass of 2 feet in diameter! It is to be hoped that these glasses may turn out to be remarkable for quality as well as size. + Dr. Robinson, in a brief account of Lord Rosse's optical labours, delivered at the meeting of the British Association, held at Cork in 1843, has remarked that "between the spherical and parabolic figures, the extreme difference is so slight, even in the telescope of 6-feet aperture, that if the two surfaces touched at their vertex, the distance at the edge would not amount to the 15th of an inch, a space which few can measure, and none without the microscope. This statement may give the reader some idea of the exquisite delicacy of the operation necessary for giving the speculum its true parabolic figure. pose of employing it in physical observations of the celestial bodies. At the meeting of the British Association, which was held at Edinburgh in 1850, it was agreed to memorialise the Government upon the advantages which would accrue to astronomical science from the practical adoption of such a scheme. It is to be hoped that, ere long, steps will be taken towards its complete realisation. CHAPTER XXI. Origin of Stellar Astronomy.-Physical Changes observed in the Starry Regions.— Disappearance of Stars from the Heavens.-New Stars.-Stars of Variable Lustre.Photometric Researches on the Stars.-Attempts to Determine their Apparent Diameters. Space-penetrating Power of Telescopes.—Applied to ascertain the relative Distances of the Stars. Absolute Distances of the Stars determined by Photometric Principles.—Parallax of the Fixed Stars.-Early Researches on the Subject.— Modern Researches.-Bessel.-Henderson.-Struve.-Peters.-Proper Motions of the Stars.-Motion of the Solar System in Space.-Double Stars.-Discovery of their Physical Connexion by Sir William Herschel.-Methods for determining the Elements of their Orbits.-Nebulæ.-Speculations of Sir William Herschel.-Modern Researches on the Subject.-Sir John Herschel.-The Earl of Rosse.-Early Speculations on the Milky Way.-Theory of Wright.-Observations of Sir William Herschel. -Speculations of that Astronomer on the breaking up of the Milky Way.-Researches of Struve on the Distribution of the Stars in Space.-Gauges of Sir John Herschel in the Southern Hemisphere.—Speculations of M. Struve on the Extinction of Light in its Passage through Space. THE aspect of the starry firmament furnishes one of the most glorious spectacles which nature displays throughout the entire range of her widelydiversified empire. The multitude of luminaries with which the blue vault of heaven appears bespangled, the endless variety of lustre which they exhibit, the striking configurations which they form, and the calm regularity with which they pursue their nocturnal courses-all conspire together in awakening a feeling of adoration in the most listless spectator, while at the same time they are eminently calculated to excite the enquiries of the more rational observer of physical phenomena. Attention could not long have been directed to the appearance of the starry heavens, before it was discovered that, except in a few instances, the entire multitude of luminaries constantly maintained the same relative position. Hence they received the appellation of Fixed Stars, an expression, however, which is now found to be inaccurate, since the researches of modern astronomers have served to demonstrate that many of the stars are affected by very minute movements, while at the same time it can hardly be doubted that they are all subject to a similar influence. During the early period of astronomical science, the fixed stars were observed merely on account of their utility in forming reference points, by means of which the apparent positions of the planetary bodies might be determined with greater accuracy, and the laws of their movements hence ascertained. Indeed, the Ptolemaic astronomy, by making the whole sphere of the stars perform a complete revolution round the earth once in every twenty-four hours, assigned to those luminaries a very subordinate place in the physical universe. When the immortal Copernicus restored the true system of the world, the stars assumed their just dignity as vast bodies placed at an immeasurable distance from the earth; but it seemed hopeless ever to arrive at any knowledge respecting their physical constitution. At length the invention of the telescope, about the beginning of the seventeenth century, by removing the veil which had so long concealed those remote orbs from the scrutinies of the human mind, gave an unexpected impulse to researches of this nature. From the epoch of Galileo's brilliant discoveries in the celestial regions, may be said to date the origin of Stellar Astronomy, a branch of enquiry which has ever since attracted more or less attention, and which, in recent times especially, has excited an intense degree of interest. That the stars are so many suns shining by their own light, is an opinion which seems to have been entertained by some of the ancient philosophers. This view of their nature came to be universally adopted upon the establishment of the true system of the world by Copernicus, since it was considered to be improbable in the highest degree, that bodies shining by reflected light would be visible at the earth, when at the same time their distance was so immense, that they did not exhibit any sensible change of position even when viewed at the opposite extremities of a diameter of the terrestrial orbit. The number of stars visible to the naked eye, is found to amount only to a few thousands; for although at first sight they seem to be innumerable, yet a closer scrutiny of the heavens soon serves to convince the observer that he has been labouring under a delusion in this respect. But what the unaided organ of vision merely suggests to the imagination, without affording any substantial grounds of belief in its existence, an examination of the starry heavens with the telescope has proved to be a reality. Multitudes of stars are then perceived, which were invisible to the naked eye. The number also thus disclosed to view, continually augments with each successive increase of the optical power of the instrument, until the imagination is at length absolutely overwhelmed with the countless myriads of suns, and systems of suns, which are found to people the immensity of space. The ancient philosophers supposed the celestial bodies to be essentially incorruptible and eternal, and hence maintained that they were not subject to physical changes, as in the case of bodies at the surface of the earth. The progress of astronomical science, however, has served amply to demonstrate that those brilliant orbs are not exempt from the great law of mutation, to which all the other objects of the created universe appear to be liable. Exclusively of the evidence afforded by the observations of astronomers on the physical constitution of the sun, there are various stellar phenomena of a highly-interesting nature, which concur in establishing this important fact, A brief allusion to some of them may not perhaps be unacceptable to the reader. In some instances, stars which have shone for ages in the firmament, have ceased to be visible. A comparison of catalogues of the stars constructed for different epochs, would indeed seem to indicate that a great number of such objects that were formerly visible in the celestial sphere, have totally vanished; but it is probable that in most of these cases the discordance is attributable to the imperfection of the earlier observations of astronomers. Stars, however, have been mentioned by Montanari*, Maraldi t, and Sir William Herschel, respecting whose extinction there hardly exist grounds for entertaining any reasonable doubt. Phil. Trans., 1671, p. 2202. + Mém. Acad. des Sciences, 1709, p. 40. Phil. Trans., 1792, p. 26. Various new stars are recorded in history, which after having shone with great splendour for a short time, then gradually faded away until they ceased to be visible. Thus Pliny asserts, that the appearance of a new star in the time of Hipparchus, about the year 180 A.D., was the occasion of inducing that astronomer to construct his famous catalogue of the stars. A new star is said to have appeared in the reign of the Emperor Honorius, about the year 390 A.D.; also one in the reign of the Emperor Otho, about the year 945; and again a phenomenon of a similar nature in the year 1264. In the year 1572, there appeared a splendid new star in the constellation of Cassiopeia, which is memorable for being the earliest of such phenomena respecting which any authentic particulars have been recorded by astronomers. This star was observed by Tycho Brahé, who, in a long work, written expressly on the subject, has given a detailed account of the various circumstances connected with its appearance; including a critical discussion of the speculations of his contemporaries on its probable origin. It was first seen by him on the evening of the 11th of November, 1572. It then surpassed in lustre the brightest of the fixed stars, and was even more brilliant than the planet Jupiter, although then in opposition and near perihelion. It almost rivalled Venus, and, like that planet, was seen by some persons even in the daytime. During the remaining part of November, it continued to shine with undiminished lustre; but it subsequently began to decline, and it gradually grew fainter, until at length, in the month of March, 1574, it ceased to be visible. The colour of this extraordinary object underwent a succession of changes during the period of its appearance. When it first became visible, it shone with a bright white light, like Venus or Jupiter. It then acquired a yellowish tinge; afterwards became ruddy, like Mars or Aldebaran; and finally exhibited a leaden hue, like the planet Saturn *. Tycho Brahé supposed it to be generated from the ethereal substance of which he imagined the Milky Way to be composed, and to have been afterwards dissipated by the light of the sun and the other stars, or to have dissolved spontaneously from some internal causet. A more modern hypothesis has referred the origin of the phenomenon to some vast combustion, an explanation which receives some degree of support from the gradual change of colour which the light of the star exhibited. A few years after the close of the sixteenth century, another splendid new star burst forth in the constellation of Serpentarius. In this instance the phenomenon was witnessed by many eminent astronomers, including Kepler, who wrote an interesting dissertation on the subject of its appearancet. It was first seen by that astronomer on the 17th of October, 1604. It surpassed in brightness the stars of the first magnitude, as well as the planets Mars, Saturn, and Jupiter, all of which were in its vicinity. Like the star of 1572, it began to decline soon after its appearance, and finally ceased to be visible between October, 1605, and February, 1606. Kepler was of opinion that it was generated from an ethereal substance, not confined exclusively to the region of the Milky Way, as Tycho Brahe had supposed in the case of the star of 1572, but pervading all space. In connexion with this explanation of the origin of the star, he remarked, that the luminous ring observed around the dark body of • Progymnasmata, p. 297. + Ibid., p. 795. "De Stella Nova in Pede Serpentarii," 4to, Pragæ, 1606. |