WEEKLY EVENING MEETING, Friday, May 18, 1866. SIR HENRY HOLLAND, BART., M.D. D.C.L. F.R.S. President, in the Chair. The REV. CHARLES PRITCHARD, F.R.S. PRESIDENT OF THE ROYAL ASTRONOMICAL SOCIETY. On the Telescope: its Modern Form, and the Difficulties of its Construction. 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; and this appears to be the first record of the existence of a lens. It had evidently been used for the purpose of magnifying the microscopic cuttings on the gems necessary to produce the intended effect. 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. 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 after. wards 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 this discovery, 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. 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 contrivance materially increased the field of view, and with such a telescope, now extended to the lengths of twelve feet and of twenty-three 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. The field of view was again so greatly increased by this invention that Huyghens was enabled to make use of telescopes of the length of even 120 to 160 feet. The enormous length which was at this time necessary to assign to a telescope in order to produce what is now considered 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, combined at the same time with very narrow limits to the size of the object-glass, were principally two, independently of the impossibility of procuring homogeneous glass of large dimensions. First, the effect of a lens upon a pencil of light incident 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 of producing 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 conclusion that it was not possible to obtain optical or magnifying power without, at the same time, producing the dispersion of colour, which, as we have seen, is inconsistent with clear definition in telescopes of manageable dimen sions. 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 (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 powerful plate lens acts, from the nature of the material, more powerfully in refracting the middle or green portion of the spectrum than can be 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 30th part of that of the object-glass. In the best modern telescopes this defect is left to its fate. 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 and 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. Mr. The necessity of employing 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 object-glass. Generally speaking, any lens, of which the surfaces are spherical, is much more powerful towards the margin than are the parts of it near to 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 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. By the application of these 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 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 investigation of the best methods of obtaining 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 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 arise in the construction of the aplanatic object-glasses of modern telescopes. We may consider then that the once difficult question of the removal of spherical aberration in an object-glass as now practically solved, and that too in a manner which requires but little further trouble than the inspection of a set of tables. There does, however, still exist a practical difficulty, where theoretically there was, or even now is, supposed to be none. It is almost universally asserted in treatises upon the subject, that in order to produce an achromatic combination nothing further is required than to make the focal lengths of the two lenses proportional to the dispersive powers of the materials of which they consist. Practically, and even theoretically, this is not the case; but, on the contrary, the proper ratio of the focal lengths of the two lenses is perceptibly influenced by the forms or curvatures of the lenses themselves. It is herein that the eye and the skill of the optician are required; and (perhaps unexpectedly) it is in this direction that we are to look for one of the weakest and most troublesome elements in the construction of the object-glasses of telescopes. Such, however, has been the improvement in the manufacture of glass in England, and such is the ability of modern English artists, that there is good reason for believing they have now regained the ancient supremacy, which in this respect existed in the time of Dollond. [C. P.] WEEKLY EVENING MEETING, Friday, May 25, 1866. Sir HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. President, in the Chair. A. S. HERSCHEL, Esq. B.A. On the Shooting-stars of the years 1865–6, and on the Probability of the Cosmical Theory of their Origin. ATTENTION was recently directed by Professor Newton, of Yale College, U.S., to the probability, on well-considered grounds, that in the current year, 1866, a prodigious flight of meteors, the most imposing of its kind, and visible over a large area of the earth's surface, will make its appearance-perhaps for the last time in the present century-either on the morning of the 13th, or on the 14th of November. The rare opportunity thus afforded of deciding some important questions in the theory of shooting-stars makes it a matter of special interest for persons skilled in such accurate observations, to watch for its return on each of the mornings named (wherever practicable, between one and two o'clock†), to obtain the necessary data. The phenomenon at its maximum was seen by Humboldt, at Cumana, on the morning of the 12th of November, 1799; and again by Dr. Denison Olmsted, in its greatest brilliancy, at Newhaven, U.S., on the morning of the 13th of November, 1833. Olmsted was the first to sum up, in the following general language, the chief characteristics of the display : 1. The number, especially of bright meteors, is much larger than usual. 2. An uncommonly large proportion leave luminous trains. 3. They proceed, with few exceptions, from a common centre in some part of the constellation Leo. 4. They are seen from midnight to sunrise, and in greatest abundance between three and four A.M. *American Journal of Science,' 2nd series, vol. xxxvii. p. 377; and vol. xxxviii. p. 53. The object in restricting the watch to a particular hour is, that as many meteors as possible may be simultaneously observed at distant places. |