Such telescopes as I have alluded to above, are among the largest which have yet been made on the achromatic principle; they are, of course, comparatively rare, and can be afforded only at a very high price. Few of the object glasses in the telescopes to which I have referred, would be valued at less than 200 Guineas, independently of the tubes, eye pieces and other apparatus with which they are fitted up. It is so difficult to procure large discs of flint glass for optical purposes, to produce the requisite curves of the different lenses, and to combine them together with that extreme accuracy which is requisite, that when a good compound lens of this description is found perfectly achromatic, the optician must necessarily set a high value upon it; since it may happen that he may have finished half a dozen before he has got one that is nearly perfect. The more common sizes of achromatic telescopes for astronomical purposes, which are regularly sold by the London opticians, are the following:—
1. The 2½ feet Achromatic.—This telescope has an object glass 30 inches in focal length, and 2 inches clear aperture. It is generally furnished with two eye pieces, one for terrestrial objects, magnifying about 30 or 35 times, and one for celestial objects with a power of 70 or 75 times. It might be furnished with an additional astronomical eye-piece—if the object glass be a good one, so as to produce a power of 90 or 95 times. With such a telescope, the belts and satellites of Jupiter, the phases of Venus and the ring of Saturn may be perceived; but not to so much advantage as with larger telescopes. It is generally fitted up either with a mahogany or a brass tube, and is placed upon a tripod brass stand, with a universal joint which produces a horizontal and vertical motion. It is packed, along with the eye-pieces, and whatever else belongs to it, in a neat mahogany box. Its price varies, according as it is furnished with an elevating rack or other apparatus.
The following are the prices of this instrument as marked in the catalogue of Mr. Tulley, Terrett’s Court, Islington, London.
| £ | s. | d. | |
|---|---|---|---|
| 2½ feet telescopes, brass mounted on plain pillar and claw stand, with one eye piece for astronomical purposes, and one for land objects, to vary the magnifying power, packed in a mahogany box | 10 | 10 | 0 |
| Ditto, ditto, brass mounted on pillar and claw stand, with elevating rack, 1 eye piece for astronomical purposes, and 1 for land objects, to vary the magnifying power, packed in a mahogany box | 12 | 12 | 0 |
The following prices of the same kind of telescope are from the catalogue of Messrs. W. and, S. Jones, 30, Lower Holborn, London.
| £ | s. | d. | |
|---|---|---|---|
| The improved 2½ feet achromatic refractor, on a brass stand, mahogany tube, with three eye pieces, two magnifying about 40 and 50 times for terrestrial objects, and the other about 75 times for astronomical purposes, in a mahogany case | 10 | 10 | 0 |
| Ditto, ditto, the tube all brass, with three eye pieces | 11 | 11 | 0 |
| Ditto, ditto, with vertical and horizontal rack work motions | 15 | 15 | 0 |
2. The 3½ feet Achromatic Telescope.—The object glass of this telescope is from 44 to 46 inches focal length, and 2¾ inches diameter. It is generally furnished with four eye-pieces, two for terrestrial and two for celestial objects. The lowest power for land objects is generally about 45, which affords a large field of view, and exhibits the objects with great brilliance. The other terrestrial power is usually from 65 to 70. The astronomical powers are about 80 and 130; but such a telescope should always have another eye-piece, to produce a power of 180 or 200 times, which it will bear with distinctness, in a serene state of the atmosphere, if the object glass be truly achromatic. The illuminating power in this telescope is nearly double that of the 2½ feet telescope, or in the proportion of 7, 56 to 4; and therefore it will bear about double the magnifying power with nearly equal distinctness. This telescope is fitted up in a manner somewhat similar to the former, with a tripod stand which is placed upon a table. Sometimes, however, it is mounted on a long mahogany stand which rests upon the floor, (as in fig. 58.), and is fitted with an equatorial motion; and has generally a small telescope fixed near the eye end of the large tube, called a finder, which serves to direct the telescope to a particular object in the heavens when the higher powers are applied. It is likewise eligible that it should have an elevating rack and sliding tubes, for supporting the eye end of the instrument, to keep it steady during astronomical observations, and it would be an advantage, for various purposes which shall be afterwards described, to have fitted to it a Diagonal Eye Piece magnifying 40 times or upwards.
The prices of this instrument, as marked in Mr. Tulley’s Catalogue, are as follows:—
| £ | s. | d. | |
|---|---|---|---|
| The 3½ feet achromatic telescope 2¾ inches aperture, on plain pillar and claw stand, 2 eye pieces for astronomical purposes, and 1 for land objects to vary the magnifying power, packed in a mahogany box | 21 | 0 | 0 |
| Ditto, ditto, with elevating rack and achromatic finder, 2 eye pieces for astronomical purposes, and 1 for day objects to vary the magnifying power, packed in a mahogany box | 26 | 5 | 0 |
The following are the prices as marked in Messrs. W. and S. Jones’ Catalogue.