_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 3-3/4 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 5-1/2 inches diameter, and about 5-1/2 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]