It appears, however, that Dollond was not the only person who had the merit of making this discovery—a private gentleman, Mr. Chest, of Chest-hall, a considerable number of years before, having made a similar discovery, and applied it to the same purpose. This fact was ascertained in the course of a process raised against Dollond at the instance of Watkins, optician at Charing-cross, when applying for a patent. But as the other gentleman had kept his invention a secret, and Dollond had brought it forth for the benefit of the public, the decision was given in his favour. There was no evidence that Dollond borrowed the idea from his competitor, and both were, to a certain extent, entitled to the merits of the invention.
One of the greatest obstructions to the construction of large achromatic telescopes is, the difficulty of procuring large discs of flint glass of an uniform refractive density—of good colour, and free from veins. It is said that, fortunately for Mr. Dollond, this kind of glass was procurable when he began to make achromatic telescopes, though the attempts of ingenious chemists have since been exerted to make it without much success. It is also said, that the glass employed by Dollond in the fabrication of his best telescopes, was of the same melting, or made at the same time, and that, excepting this particular treasure, casually obtained, good dense glass for achromatic purposes, was always as difficult to be procured as it is now. The dispersion of the flint glass, too, is so variable, that, in forming an achromatic lens, trials on each specimen require to be made before the absolute proportional dispersion of the substances can be ascertained. It is owing, in a great measure, to these circumstances, that a large and good achromatic telescope cannot be procured unless at a very high price. Mr. Tulley of Islington—who has been long distinguished as a maker of excellent achromatic instruments—showed me, about six years ago, a rude piece of flint glass about five inches diameter, intended for the concave lens of an achromatic object glass, for which he paid eight guineas. This was before the piece of glass was either figured or polished, and, consequently, he had still to perform the delicate operation of figuring, polishing, and adjusting this concave to the convex lenses with which it was to be combined; and during the process some veins or irregularities might be detected in the flint glass which did not then appear. Some years before, he procured a disc of glass from the continent about seven or eight inches diameter, for which he paid about thirty guineas, with which an excellent telescope, twelve feet focal length, was constructed for the Astronomical Society of London. It is obvious therefore, that large achromatic telescopes must be charged at a pretty high price.
In order to stimulate ingenious chemists and opticians to make experiments on this subject, the Board of Longitude, more than half a century ago, offered a considerable reward for bringing the art of making good flint glass for optical purposes to the requisite perfection. But considerable difficulties arise in attempting improvements of this kind; as the experiments must all be tried on a very large scale, and are necessarily attended with a heavy expence. And although government has been extremely liberal in voting money for warlike purposes, and in bestowing pensions on those who stood in no need of them, it has hitherto thrown an obstruction in the way of such experiments, by the heavy duty of excise, which is rigorously exacted, whether the glass be manufactured into saleable articles or not; and has thus been instrumental in retarding the progress of improvement and discovery. It would appear that experiments of this kind have been attended with more success in France, Germany, and other places on the continent, than in Britain; as several very large achromatic telescopes have been constructed in those countries by means of flint glass which was cast for the purpose in different manufactories, and to which British artists have been considerably indebted; as the London opticians frequently purchase their largest discs of flint glass from Parisian agents. Guinaud, a continental experimenter, and who was originally a cabinet maker, appears to have had his labours in this department of art crowned with great success. Many years were employed in his experiments, and he too frequently, notwithstanding all his attention, discovered his metal to be vitiated by striæ, spects or grains, with cometic tails. He constructed a furnace capable of melting two cwt of glass in one mass, which he sawed vertically, and polished one of the sections, in order to observe what had taken place during the fusion. From time to time, as he obtained blocks, including portions of good glass, his practice was to separate them by sawing the blocks into horizontal sections, or perpendicular to their axes. A fortunate accident conducted him to a better process. While his men were one day carrying a block of this glass, on a hand-barrow, to a saw mill which he had erected at the Fall of the Doubs, the mass slipped from its bearers, and, rolling to the bottom of a steep and rocky declivity, was broken to pieces. Guinaud having selected those fragments which appeared perfectly homogeneous, softened them in circular moulds, in such a manner, that on cooling, he obtained discs that were afterwards fit for working. To this method he adhered, and contrived a way for clearing his glass while cooling, so that the fractures should follow the most faulty parts. When flaws occurred in the large masses, they were removed by cleaving the pieces with wedges; then smelting them again in moulds, which give them the form of discs. The Astronomical Society of London have made trial of discs made by Guinaud, and have found them entirely homogeneous and free from fault. Of this ingenious artist’s flint glass, some of the largest achromatic telescopes on the continent have been constructed. But, it is more than twenty years since this experimenter took his flight from this terrestrial scene, and it is uncertain whether his process be still carried on with equal success.
Notices of some large Achromatic telescopes on the Continent and in Great Britain.
1. The Dorpat Telescope.—This is one of the largest and most expensive Refracting telescopes ever constructed. It was made by the celebrated Fraunhofer of Munich for the observatory of the Imperial University of Dorpat, and was received into the observatory by Professor Struve in the year 1825. The aperture of the object glass of this telescope is 9½ English inches, and its solar focal length about fourteen feet, the main tube being thirteen French feet exclusive of the tube which holds the eye pieces. The smallest of the four magnifying powers it possesses, is 175, and the largest 700, which, in favourable weather, is said to present the object with the utmost precision. ‘This instrument,’ says Struve, ‘was sold to us by Privy-Counsellor Von Utzchneider, the chief of the optical establishment at Munich, for 10,500 florins, (about £950 sterling), a price which only covers the expenses which the establishment incurred in making it.’ The frame work of the stand of this telescope is of oak inlaid with pieces of mahogany in an ornamental manner, and the tube is of deal veneered with mahogany and highly polished. The whole weight of the telescope and its counterpoises is supported at one point, at the common center of gravity of all its parts; and though these weigh 3000 Russian pounds, yet, we are told that this enormous telescope may be turned in every direction towards the heavens with more ease and certainty than any other hitherto in use. When the object end of the telescope is elevated to the zenith, it is sixteen feet four inches, Paris measure, above the floor, and its eye end in this position is two feet nine inches high. This instrument is mounted on an Equatorial stand, and clock work is applied to the Equatorial axis, which gives it a smooth and regular sidereal motion, which, it is said, keeps a star in the exact center of the field of view, and produces the appearance of a state of rest in the starry regions, which motion can be made solar, or even lunar, by a little change given to the place of a pointer, that is placed as an index on the dial plate. Professor Struve considers the optical powers of this telescope superior to those of Schröeter’s twenty-five feet reflector, from having observed σ Orionis with fifteen companions, though Schröeter observed only twelve, that he could count with certainty. Nay, he seems disposed to place it in competition with the late Sir W. Herschel’s forty feet reflector. The finder of this telescope has a focal distance of 30 French inches, and 2-42 aperture.
2. Sir James South’s Telescope.—About the year 1829, Sir J. South, President of the London Astronomical Society, procured of M. Cauchoix of Paris, an achromatic object glass of 112/10 inches, clear aperture, and of 19 feet focal length. The flint glass employed in its construction was the manufacture of the late Guinaud le Pere, and was found to be absolutely perfect. The first observation was made with this telescope, while on a temporary stand, on Feb. 13, 1830, when Sir J. Herschel discovered with it a sixth star in the trapezium in the nebula of Orion, whose brightness was about one third of that of the fifth star discovered by Struve, which is as distinctly seen as the companion to Polaris is in a five feet achromatic. Sir James gives the following notices of the performance of this instrument on the morning of May 14, 1830. ‘At half past two, placed the 20 feet achromatic on the Georgium Sidus, saw it with a power of 346, a beautiful planetary disc; not the slightest suspicion of any ring, either perpendicular or horizontal; but the planet three hours east of the meridian, and the moon within three degrees of the planet.’ At a quarter before three, viewed Jupiter with 252 and 346, literally covered with belts, and the diameters of his satellites might have been as easily measured as himself. One came from behind the body, and the contrast of the colour with that of the planet’s limb was striking. At three o’clock viewed Mars. The contrast of light in the vicinity of the poles very decided. Several spots on his body well and strongly marked—that about the south pole seems to overtake the body of the planet, and gives an appearance not unlike that afforded by the new moon, familiarly known as ‘the old moon in the new moon’s arms.’ Saturn has been repeatedly seen with powers from 130 to 928 under circumstances the most favourable; but not any thing anomalous about the planet or its ring could even be suspected. This telescope is erected on an Equatorial stand at Sir J. South’s observatory, Kensington.
3. Captain Smyth’s Telescope in his private observatory at Bedford.—This Achromatic telescope is 8½ feet focal length, with a clear aperture of 59/10 inches worked by the late Mr. Tulley, Senior, from a disk purchased by Sir James South at Paris. It is considered by Captain Smyth to be the finest specimen of that eminent optician’s skill, and, it is said, will bear with distinctness, a magnifying power of 1200. Its distinctness has been proved by the clear vision it gives of the obscure nebulæ, and of the companions of Polaris, Rigel, α Lyræ, and the most minute double stars—-the lunar mountains, cavities and shadows under all powers—the lucid polar regions of Mars—the sharpness of the double ring of Saturn—the gibbous aspect of Venus—the shadows of Jupiter’s satellites across his body, and the splendid contrast of colours in α Hercules, γ Andromedæ and other superb double stars.
Other large Achromatics.—Besides the above, the following, belonging to public observatories and private individuals, may be mentioned. In the Royal observatory at Greenwich, there is an Achromatic of 10 feet focal distance, having a double object glass 5 inches diameter, which was made by Mr. Peter Dollond, and the only one of that size he ever constructed. There is also a 46 inch achromatic, with a triple object glass 3¾ inches aperture, which is said to be the most perfect instrument of the kind ever produced. It was the favourite instrument of Dr. Maskelyne, late Astronomer Royal, who had a small room fitted up in the observatory for this telescope. The observatory, some years ago erected near Cambridge, is perhaps the most splendid structure of the kind in Great Britain. It is furnished with several very large achromatic telescopes on Equatorial machinery: but the Achromatic telescope, lately presented to it by the Duke of Northumberland, is undoubtedly the largest instrument of this description which is to be found in this country. The object glass is said to be 25 feet focal distance, and of a corresponding diameter, but as there was no access to this instrument at the time I visited this observatory, nearly six years ago, I am unable to give a particular description of it. In the Royal Observatory at Paris, which I visited in 1837, I noticed, among other instruments, two very large Achromatic telescopes which, measuring them rudely by the eye—I estimated to be from 15 to 18 feet long, and the aperture at the object end, from 12 to 15 inches diameter. They were the largest achromatics I had previously seen; but I could find no person in the observatory at that time, who could give me any information as to their history, or to their exact dimensions, or powers of magnifying.[21]
The Rev. Dr. Pearson, Treasurer to the Astronomical Society of London, is in possession of the telescope formerly alluded to, made by Mr. Tulley, of twelve feet focal distance and seven inches aperture, which is said to be a very fine one. The small star which accompanies the pole star, with a power of a 100, appears through this telescope, as distinct and steady as one of Jupiter’s satellites. With a single lens of 6 inches focus, which produced a power of 24 times, according to the testimony of an observer who noticed it—the small star appeared as it does in an achromatic of 3 inches aperture, which shows the great effect of illuminating power in such instruments. Mr. Lawson, a diligent astronomical observer in Hereford, possesses a most beautiful achromatic telescope of about 7 inches aperture, and 12 feet focal distance, which was made by one of the Dollonds, who considered it as his chief d’oeuvre. It is said to bear powers as high as 1100 or 1400; and has been fitted up with mechanism devised by Mr. Lawson himself, so as to be perfectly easy and manageable to the observer, and which displays this gentleman’s inventive talent. In several of his observations with this instrument, he is said to have had a view of some of the more minute subdivisions of the ring of Saturn. A very excellent achromatic telescope was fitted up some years ago by my worthy friend William Bridges, Esq., Blackheath. Its object glass is 5½ inches diameter, and about 5½ feet focal length. It is erected upon Equatorial machinery, and placed in a circular observatory which moves round with a slight touch of the hand. The object glass of this instrument cost about 200 Guineas, the equatorial machinery on which it is mounted cost 150 Guineas, and the circular observatory in which it is placed about 100 Guineas; in all 450 Guineas. Its powers vary from 50 to 300 times.[22]