Astronomical Clock: not very first-class; price low. INSTRUMENTS, &c. WANTED. [65] Equatorial Stand, for a 5 ft. Refractor: graduated circles. [35] [71] [82] Monthly Notices of the Royal Astronomical Society, Vol. 3. [26] Monthly Notices: Vols. 3, 4, 5, and 7. Bartholomew Prescot on the Universe, published 1823: a copy wanted, in good condition. [84] SATURN.-In a note just received, Mr W. L. Banks of Ealing says "I have observed the outer satellite at intervals during fifty-five nights of his present revolution; a larger number of nights than I ever did before, as it is usually invisible in the North East quarter of the orbit." In order to save the expense of Postage, Subscriptions, as a rule, are acknowledged in the number of the Register for the month following that in which they are received, if not sent later than the 20th, after which they will be acknowledged in the next following number. LIST OF SUBSCRIBERS-Names received since our last number. TO CORRESPONDENTS. Several communications are unavoidably postponed. ERRATUM.-In No. 42, page 163, line 7, for 153, read 15s. QUATORIAL TELESCOPE and STAND, by T. SLATER, Telescope has been used in a private observatory three years, and its performance on close double stars guaranteed: it has been removed in consequence of its late owner having purchased of the same maker an object glass of 16 in. diameter. [80] 10 BE SOLD, a great bargain, a 7-inch ACHROMATIC TELEbrass-mounted.-Eye-pieces, eight negative, 90 to 600; four positive, one day, one nebula, one comet, small diagonal, large diagonal adapter, sun prism, transit, three Barlow lenses: Ramsden's micrometer by Berge, two reading micrometers, a position micrometer with two eye-pieces, graduated on platina, by Troughton & Simms, with slipping piece and illuminating apparatus.-On large garden Equatorial Stand, without circles; with steadying rods, and 5 in. circle on silver with level, adapted to the telescope to find declination when on the meridian. [81] TO BE 1s in Boots 10 BE SOLD, a 12 in. SILVERED GLASS SPECULUM, greatest ease. WA [78] ANTED-INSTRUMENTS FOR THE ILLUSTRATION OF ASTRONOMICAL LECTURES.-Address, ASTER, 35, Somerset Street, Kingsdown, Bristol. [83] The Astronomical Register is intended to appear at the commencement of each month: the Subscription, (including Postage) is fixed at Three Shillings per Quarter, payable in advance, by postage stamps or otherwise. The pages of the Astronomical Register are open to all suitable communications: Letters, Articles for insertion, &c., must be sent to the Editor, MR.S.GORTON, Stamford Villa, Downs Road, Clapton, N.E., not later than the 15th of the month. The Astronomical Register. No. 44. AUGUST. 1866. THE TELESCOPE: A Paper read at the Royal Institution, by the Rev. C. PRITCHARD, P.R.A.S., May 18, 1866. IN the museum at Naples, among other articles exhumed from the volcanic mud of Herculaneum, will be found the contents of a lapidary's shop; the visitor may there see the half-finished gems, and the tools with which they were being engraved at the moment of the workmen's fright. By the side of them is a piece of glass, rudely shaped into a convex form; it had evidently been used for the purpose of magnifying the microscopic cuttings on the gems, necessary to produce the intended effect. This appears to be the first record of the existence of a lens. Thirteen hundred years after this, spectacle-glasses had become somewhat common in Europe, and it was by a combination of these that one Hans Lippershey, in the year 1608, at Middleburg, in the Netherlands, invented a telescope, by means of which "distant objects were brought nearer." Several of these instruments, very early in the year 1609, were provided in a binocular form, for and at the expense of the States-General of the Netherlands; but there is no record of their having been directed to any astronomical object. The material out of which the object-lenses were ground seems to have been quartz. The cost of each binocular was about 751. It was originally intended to keep the construction of these instruments secret, for strategical purposes ! When the minister of Henry IV. of France proposed to send one of these telescopes to his master, because "it enables one to see at a great distance," that monarch returned the following characteristic reply: "I shall see with pleasure the glasses which you mention in your letter, though at present I am more in want of such that can show me things near me."-Journal of the Royal Institution, 1830. In May 1609, Galileo, at Venice, heard of this new invention, without, so far as is known, any precise intimation of the method by which the effect was produced. He hastened home to Padua, and on the day after his arrival he produced the form of telescope which still bears his name. His first instrument magnified but three times. Very shortly afterwards he constructed a second telescope, possessing a power of about six linear. With this he discovered the satellites of Jupiter, in January 1610. He also observed spots in the sun, and mountains in the moon. Very soon after these discoveries, Galileo succeeded in constructing a telescope which magnified about thirty-three times linear; and with this he discovered the long-suspected phases of the planet Venus, thereby completing the proof which was still wanting of the truth of the Copernican system. There can be little doubt that the intellectual convictions which necessarily followed upon these discoveries, emancipated mankind from the thraldom of the dogmas of the Aristotelians and the Schoolmen, while, at the same time, they consigned Galileo to the persecutions of the Inquisition, and deprived him ultimately of his personal liberty. It is remarkable that, so far as the means then in existence permitted, Galileo carried his particular form of telescope to its furthest practicable limit of perfection. The last use which he made of his instrument was to observe and partly explain the Librations of the Moon, but no sooner was this accomplished, than, to use the touching language of Castelli, “the noblest eye that nature ever made was darkened," and blindness put an effectual stop to all further astronomical researches. This occurred in 1636, at a time when the rigorous ecclesiastical surveillance over Galileo had been so far relaxed that he was permitted to receive his friends in his villa at Arcetri, near Florence. It was here that John Milton visited him in 1638, but the only record of this most suggestive visit is that given by the great poet in these few words: "There (in Italy) it was that I found and visited the famous Galileo, grown old, a prisoner to the Inquisition, for thinking in astronomy otherwise than the Franciscan and Dominican licensers thought." The impression, however, that was made on Milton's mind, judging from his repeated allusions to the great Florentine in his poetical works, must have been exceedingly great; it may here suffice to select a single instance :— "The broad circumference Hung on his shoulders like the Moon, whose orb Or in Valdarno, to descry new lands, Rivers, or mountains, in her spotty globe." Par. Lost, i. 290. Milton was born in the year of the invention of the telescope, and Galileo died in the year after Newton's birth. For some forty years the telescope remained in the form in which Galileo left it. In 1656, Huyghens, at the Hague, substituted two convex lenses in contact for the eye-glass instead of the single concave lens employed by Galileo. This adaptation materially increased the field of view, and with such a telescope, now extended to the lengths of twelve feet and of twentythree feet, Huyghens discovered Saturn's ring, supposed by Galileo to consist of two small spheres, one on either side of the planet. He also discovered one of the satellites of Saturn, and he contrived that admirable form of eye-piece called the Huyghenian, which, with no material improvement, is still in constant use at the present day. Three important results are at once secured by the Huyghenian eye-piece: first, the field of view is greatly augmented; secondly, the spherical aberration is less than that of a single eye lens, or even than that of two-eye lenses in contact; thirdly, the combination itself is achromatic. Armed with this eye-piece, Huyghens extended the virtual length of his telescope, though now without the intervention of a tube, to 120 and 160 feet; but it is curious to remark that the service of a good lantern was now called into requisition, in order that the observer might see the position of the distant object-glass. The enormous length which was at this time necessary to assign to a telescope in order to produce what is now considered only a small amount of amplifying power, put a natural and very confined limit to the practical application of the instrument. The causes which necessitated this great length of the instrument, and which, at the same time, restricted the size of the object-glass within very narrow limits, were principally two, independently of the possibility of procuring homogeneous glass of large dimensions. First, the effect of a lens upon a conical pencil of light incident directly and centrally upon it, is to spread it out into a coloured circle, of which the diameter is, with ordinary glass, about one-fiftieth part of the diameter of the object-glass. This defect-if the consequence of a natural law can be properly termed a defect-evidently set a natural limit not only to the size of the object-glass, but to the amount of magnifying power applicable to the eye-piece; thus necessitating the twofold inconvenience of contracting the size of the object-glass, and at the same time of obtaining power in the telescope by the increase of its length. Newton, by unfortunately making use of a certain species of Venetian glass of low specific gravity, and very much resembling water in its optical properties, came to the erroneous conclusion that it was not possible to obtain optical or magnifying power without, at the same time, producing that dispersion of colour which, as we have seen, is inconsistent with clear definition in telescopes of manageable dimensions. This unfortunate mistake on Newton's part discouraged for nearly a century all further attempts to improve the object-glass for telescopes. In 1758, John Dollond, by experimenting with glass of a different character to that employed by Newton, discovered the source of that great philosopher's mistake. He found that a lens of the ordinary heavy metallic glass of his day, called flint-glass, dispersed the colour of a pencil of light about as much as a lens of crown or plate-glass, possessing double the general deflective or magnifying power of the former. Hence, by combining a convex lens of plate-glass with a concave lens of flint-glass, but possessing only half the power of the former (in this case a diminishing power), he obtained a combination which was capable of forming a nearly colourless image. We say nearly colourless, because the plate lens acts, from the nature of the material, more powerfully in refracting the middle or green portion of the spectrum than is recovered by the contrary action of the correcting or flint concave lens. Hence, there remains behind an uncorrected or residuary spectrum of about th of the breadth of the original spectrum produced by the convex plate lens-that is to say, a pencil of white light is now dispersed over a circle whose diameter is about th part of that of the object-glass. In the best modern telescopes this defect is left to its fate. The actual diameter of this circle of residuary colour depends very much on the materials of the two lenses. It is very desirable that the attention of opticians be directed to this circumstance, because in the course of Mr. Pritchard's experiments he found the diameter of this circle varied in the specimens of glass before him, from th to th of the diameter of the primary circle of chromatic dispersion of the crown lens-a difference which would very plainly disclose itself in an object-glass of large dimensions. Mr. Cooke is at this moment engaged in the construction of an object-glass twenty five inches in diameter, by far the largest ever yet attempted. The diameter of the circle of chromatic diffusion in this magnificent object-glass, when completed, cannot be less than about th of an inch unless, therefore, some secondary combination is introduced, this circumstance will unavoidably prevent the employment of any powerful eye-piece. We are thus, then, and after all, brought back again to one of the original difficulties which limited the dimensions of the object-glass in the days of Galileo and Huyghens, but with this important difference, that whereas the latter philosopher was confined to glasses of six inches diameter, and one hundred and twenty feet focal length, Dollond's discovery has extended the dimensions of the object-glass beyond twenty inches, and has reduced the focal length within the manageable limits of fourteen or fifteen apertures. What is now wanted is to discard the modern heavy flint glass, and to use the present crown as the corrective of some new glass of lower dispersive power, perhaps yet to be invented. The necessity of employing very small object-glasses was thus satisfactorily removed by the discovery of Dollond in 1758. Nevertheless, there still remains another serious cause of imperfection in the compound objectglass. Generally speaking, any lens, of which the surfaces are spherical, is much more powerful towards the margin than are the parts of it near the centre. A pencil of light, incident on the whole aperture of a lens, will, in general, be diffused over a circle whose diameter bears a very appreciable ratio to the thickness of the lens. Happily, however, the actions of a convex and of a concave lens are in this respect in opposite directions, and hence they have, when combined, a tendency to correct or compensate the spherical aberrations of each other. Still more happily, the amount of this spherical aberration depends very materially on the relative curvatures of the two surfaces of the lens. Without altering the focal length of a lens, it is quite possible very seriously to alter the amount of the spherical aberration. For instance, in lenses of any material, the aberration for parallel rays of a plano-convex lens is four times that of a lens of equal power, where the curvature of the side facing the incident light is three times that of the other surface. This remarkable effect arises from the circumstance that although, upon the whole, the same total deviation of the light is produced in the two lenses, the distribution of the amount of deviation to be produced by each surface in its turn is very different. Speaking roughly, and by way of illustration of a principle, it may be said that the amount of the aberrational error at each surface depends (cæteris paribus) on the cube of the deviation produced at that surface; and inasmuch as in the case of the plano-convex lens the whole deviation is produced at the second surface, whereas in the other lens of equal power the deviations at the two surfaces are equal, it follows that the aberrational error at each of its two surfaces is th of that produced at the second surface of the plano-convex lens; that is to say, the total aberration of the latter is four times that of the former. By the application of similar principles it has become comparatively easy to produce a combination free both from primary chromatic and from spherical aberration. The correction of the colour by means of a concave flint lens depends, speaking roughly and in general, on its focal length, and not upon the relative curvatures of its surfaces; consequently these relative curvatures can be altered until those are found which balance the spherical aberration of the convex plate-glass lens. The algebraic investigation of the best methods of obtaining the exact amount of these appropriate relative curvatures is attended with extreme labour and much difficulty, and has occupied the thoughts of a long succession of accomplished mathematicians. The main source of difficulty has been supposed to arise from the practical necessity of taking the thicknesses of the lenses into the account. Mr. Pritchard, however, has recently shown that the thicknesses of the two lenses have a tendency to compensate one another in the amounts of spherical aberration which they respectively either introduce or remove, and he has demonstrated that tables constructed on the principle of neglecting the thicknesses are practically applicable to all such cases as ordinarily |