That reflecting telescopes of the descriptions now stated are original in their construction, appears from the uniform language of optical writers, some of whom have pronounced such attempts to be altogether impracticable. Sir David Brewster, one of the latest and most respectable writers on this subject, in the ‘Edinburgh Encyclopedia’ art optics, and in the last edition of his appendix to ‘Ferguson’s Lectures,’ has the following remarks:—‘If we could dispense with the use of the small specula in telescopes of moderate length, by inclining the great speculum, and using an oblique, and consequently a distorted reflection, as proposed first by La Maire, we should consider the Newtonian telescope as perfect; and on a large scale, or when the instrument exceeds 20 feet, it has undoubtedly this character, as nothing can be more simple than to magnify, by a single eye-glass, the image formed by a single speculum. As the front view is quite impracticable, and indeed has never been attempted in instruments of a small size, it becomes of great practicable consequence to remove as much as possible, the evils which arise from the use of a small speculum,’ &c.
The instruments now described have effectuated, in some degree, the desirable object alluded to by this distinguished philosopher, and the mode of construction is neither that of Sir W. Herschel’s front view, nor does it coincide with that proposed by La Maire, which appears to have been a mere hint that was never realized in the construction of reflecting telescopes of a small size. The simplicity of the construction of these instruments, and the excellence of their performance, have been much admired by several scientific gentlemen and others to whom they have been exhibited. Prior to the description of them in the Edin. Philos. Journal, they were exhibited in the Calton Hill Observatory, Edinburgh, in the presence of Professor Wallace, and another gentleman, who compared their performance with that of an excellent Gregorian. As this instrument is distinguished from every other telescope, in being used without a tube, it has been denominated ‘The aerial reflector.’
SECT. 4.—EARL OF ROSSE’S REFLECTING TELESCOPES.
This nobleman, unlike many of his compeers, has, for a considerable number of years past, devoted his attention to the pursuits of science, and particularly to the improvement of reflecting telescopes. He is evidently possessed of high mathematical attainments, combined with an uncommon degree of mechanical ingenuity. About 14 or 15 years ago, he engaged in various experiments with the view of counteracting the effects of the spherical aberration of the specula of reflecting telescopes—which imperfection, if it could be completely remedied, would render the reflecting telescope almost a perfect instrument, as it is not affected by the different refrangibility of the rays of light. His method, we believe, consisted in forming a large speculum of two or three separate pieces of metal, which were afterwards accurately combined into one—a central part which was surrounded by one or two rings ground on the same tool. When the images formed by the separate pieces, were made exactly to coincide, the image of the object towards which the whole speculum was directed, was then found to be as distinct as either image had been when separate. But at the period referred to, a sufficient number of experiments had not been made to determine that his lordship had completely accomplished the object he intended.
Great interest, however, has of late been excited by the improvements which his lordship has made in the formation of specula. Sir W. Herschel never made public the means by which he succeeded in giving such gigantic developement to the reflecting telescope: and therefore the construction of a large reflector has been considered as a perilous adventure. But, according to a report of Dr. Robinson of Armagh, to the Irish academy, the Earl of Rosse has overcome the difficulties which have hitherto been met with, and carried to an extent which even Herschel himself did not venture to contemplate, the illuminating power of this telescope, along with a sharpness of definition little inferior to that of the achromatic; and it is scarcely possible, he observes, to preserve the necessary sobriety of language in speaking of the moon’s appearance with this instrument, which Dr. Robinson believes to be the most powerful ever constructed. The difficulty of constructing large specula, and of imparting to them the requisite degree of polish, has hitherto been considered so great, that from 8 to 12 inches diameter has been in general their utmost size. Indeed, except with the greatest reluctance, London opticians would not accept of orders for specula of more than 9 inches in diameter. It appears, however, that the Earl of Rosse has succeeded, by a peculiar method of moulding, in casting object-mirrors of true speculum metal of three feet in diameter, and of a weight exceeding 17 cwt. He is about to construct a telescope, the speculum of which is six feet in diameter, fifty feet focal distance, and of the weight of four tons; and from what he has already accomplished, it is not doubted that he possesses the power to carry his design into effect. These great masses of metal, which, in the hands of all other makers of specula would have been as untractable as so much unannealed flint-glass, the Earl of Rosse has further succeeded in bringing to the highest degree of polish, and the utmost perfection of curvature by means of machinery. The process is conducted under water, by which means those variations of temperature, so fatal to the finest specula hitherto attempted, are effectually guarded against. To convince Dr. Robinson of the efficacy of this machinery, the earl took the three feet speculum out of its telescope, destroyed its polished surface, and placed it under the mechanical polisher. In six hours it was taken out with a perfect new surface as bright as the original. Under the old system of hand-polishing, it might have required months, and even years, to effect this restoration. Even before achieving these extraordinary triumphs on the solid substance, his lordship had constructed a six feet reflector by covering a curved surface of brass with squares of the true speculum metal, which gave an immense quantity of light, though subject to some irregularities, arising from the number of joinings necessary in such a mosaic work. Of the performance of his lordship’s great telescope, mounted with this reflector, those who have seen it speak in terms of high admiration; but in reference to the smaller and more perfect instrument, furnished with the solid three feet speculum, the language of the Armagh astronomer assumes a tone of enthusiasm and even of sublimity. By means of this exquisite instrument, Dr. Robinson and Sir J. South, in the intervals of a rather unfavourable night, saw several new stars, and corrected numerous errors of other observers. For example, the planet Uranus, supposed to possess a ring similar to that of Saturn, was found not to have any such appendage; and those nebulæ, hitherto regarded, from their apparently circular outline, as ‘coalescing systems,’ appeared, when tested by the three feet speculum, to be very far indeed from presenting a globular appearance; numerous off-shoots and appendages, invisible by other telescopes, appearing in all directions radiating from their edges. Such discoveries, which reflect great honour on the Earl of Rosse, will doubtless have great effect on the interests of astronomical science.[27]
SECT. 5.—REFLECTING TELESCOPES WITH GLASS SPECULA.
After making a variety of experiments with aerial telescopes constructed of metallic specula of different focal lengths, I constructed a telescope on the same plan, with a concave glass mirror. Having obtained a fragment of a very large convex mirror which happened accidentally to have been broken, I caused the convex side to be foliated, or silverised, and found its focal length to be about 27 inches. This mirror, which was about 5 inches diameter, I placed in one of the aerial reflectors, instead of the metallic speculum, and tried its effects with different terrestrial eye-pieces. With a power of about 35 or 40 times, it gave a beautiful and splendid view of distant terrestrial objects—the quantity of light reflected from them, being considerably greater than when a metallic speculum was used, and they appeared on the whole well-defined. The only imperfection—as I had foreseen—consisted in a double image being formed of objects which were remarkably bright and white, such as a light-house whitened on the outside, and strongly illuminated by the sun. One of the images was bright and the other faint. This was obviously owing to the two reflections from the two surfaces of the mirror—one from the convex silverised side, and the other from the concave side next the eye, which produced the faint image—which circumstance has been generally considered as a sufficient reason for rejecting the use of glass specula in telescopes. But although very bright objects exhibited a double image, almost all the other objects in the terrestrial landscape appeared quite distinct and without any secondary image, so that a common observer could scarcely have noticed any imperfection. When the instrument, however, was directed to celestial objects, the secondary image was somewhat vivid, so that every object appeared double. Jupiter appeared with two bodies, at a little distance from each other, and his four satellites appeared increased to eight. The moon likewise appeared as a double orb, but the principal image was distinct and well-defined. Such a telescope, therefore, was not well-adapted for celestial observations, but might answer well enough for viewing terrestrial objects.
Considering that the injurious effects of the secondary image arose from the images reflected from the two surfaces being formed near the same point, and at nearly the same focal distance, I formed a plan for destroying the secondary image, or at least counteracting its effects, by forming the concavity of the mirror next the eye of a portion of a sphere different from that of the convex side which was silverised, and from which the principal image is formed. But, for a long time, I could find no opticians possessed of tools of a sufficient length of radii for accomplishing my design. At length a London working optician undertook to finish a glass speculum, according to my directions, which were, that the convex surface of the mirror should be ground on a tool which would produce a focal distance by reflection of about 4 feet; and that the concave surface should have its focal distance at about 3 feet 3 inches, so that the secondary image might be formed at about 9 inches, within the focal distance of the silverised side, and not interfere to disturb the principal image. But, either from ignorance or inattention, the artist mistook the radius for the half radius of concavity, and the speculum turned out to be only 23 inches focal distance by reflection. This mirror was fitted up as a telescope, on the aerial plan, and I found, as I expected, the secondary image completely destroyed. It produced a very beautiful and brilliant view of land objects, and even the brightest objects exhibited no double image. The mirror was nearly 5 inches in diameter, but the image was most accurately defined when the aperture was contracted to about 3 inches. It was fitted with a terrestrial eye-piece which produced a magnifying power of about 25 times. When directed to the moon, it gave a very distinct and luminous view of that orb, without the least appearance of a secondary image. But as the focal distance of the speculum was scarcely half the length I had prescribed, I did not apply to it any high astronomical powers; as I find, that these can only be applied with effect, in this construction, to a speculum of a considerable focal length. Happening to have at hand a convex lens 10 feet focal length, and 4 inches in diameter—the one side of which had been ground to a certain degree of concavity—I caused the convex side to be foliated, which produced a focus by reflection, at 13½ inches distant. To this mirror I applied terrestrial powers of 15 and 24, with considerable distinctness. The power of 15 produced a very brilliant and distinct view of land objects. Had the mirror been at least 3 times the focal length, it would have formed an excellent telescope, with the same aperture.
SECT. 6.—A REFLECTING TELESCOPE, WITH A SINGLE MIRROR AND NO EYE-PIECE.
figure 72.