| £ | s. | d. | |
|---|---|---|---|
| The 3½ feet achromatic, plain mahogany tube | 18 | 18 | 0 |
| Ditto, ditto, brass tube | 21 | 0 | 0 |
| Ditto, all in brass, with rack work motions, &c. | 26 | 5 | 0 |
| Ditto, the object glass of the largest aperture, and the rack motions on an improved principle | from 37l. 16s. to 42 | 0 | 0 |
| Ditto, fitted up with Equatorial motion, framed mahogany stand, divided altitude, and azimuth arches, or declination and right ascension circles, &c. &c. | from 60l to 80 | 0 | 0 |
This is the telescope which I would particularly recommend to astronomical amateurs, whose pecuniary resources do not permit them to purchase more expensive instruments. When fitted up with the eye pieces and powers already mentioned, and with a finder and elevating rack,—price 25 guineas—it will serve all the purposes of general observation. By this telescope, satisfactory views may be obtained of most of the interesting phenomena of the heavens, such as the spots of the sun—the mountains, vales, and caverns on the lunar surface—the phases of Mercury and Venus—the spots on Mars—the satellites and belts of Jupiter—the ring of Saturn—many of the more interesting nebulæ, and most of the double stars of the second and third classes. When the object glass of this telescope is accurately figured and perfectly achromatic, a power of from 200 to 230 maybe put upon it, by which the division of Saturn’s ring might occasionally be perceived. It is more easily managed and represents objects considerably brighter than reflecting telescopes of the same price and magnifying power, and it is not so apt to be deranged as reflectors generally are. A telescope of a less size would not in general be found satisfactory for viewing the objects I have now specified, and for general astronomical purposes. It may not be improper for the information of some readers, to explain what is meant in Mr. Tulley’s catalogue, when it is stated that this instrument has ‘one eye piece for day objects, to vary the magnifying power.’ The eye piece alluded to is so constructed, that by drawing out a tube next the eye, you may increase the power at pleasure, and make it to vary, say from 40 to 80 or 100 times; so that such a construction of the terrestrial eye piece (to be afterwards explained) serves in a great measure, the purpose of separate eye-pieces. The whole length of the 3½ feet telescope, when the terrestrial eye piece is applied, is about 4½ feet from the object glass to the first eye glass.
When the aperture of the object glass of this telescope exceeds 2¾ inches its price rapidly advances.
The following is Mr. Tulley’s scale of prices, proportionate to the increase of aperture:—
| £ | s. | d. | |
|---|---|---|---|
| 3½ feet telescopes 3¼ inches aperture, with vertical and horizontal rack work motions, achromatic finder, 3 eye pieces for astronomical purposes, and one for day objects to vary the magnifying power, packed in a mahogany box | 42 | 0 | 0 |
| Ditto, ditto, 3¾ inches diameter, mounted as above | 68 | 5 | 0 |
| Ditto, with universal Equatorial, instead of pillar and claw stand | 84 | 0 | 0 |
Here, in the one case, the increase of half an inch in the diameter of the object-glass, adds about £16. to the expense; and in the other case no less than £26. 5s. The proportion of light in those two telescopes, compared with that of 2¾ inches aperture, is as follows:—The square of the 2¾ object-glass is 7.56; that of 3¼, 10.56, and that of the 3¾, 14.06; so that the light admitted by the 3¼ compared with the 2¾ aperture, is nearly as 10 to 7; and the light admitted by the 3¾ object-glass is nearly double that of the 2¾ aperture, and will bear nearly a proportional increase of magnifying power.
3. The 5 feet Achromatic telescope. The focal length of the object-glass of this telescope is 5 feet 3 inches, and the diameter of its aperture 38/10 inches. The usual magnifying powers applied to it are, for land objects 65 times; and for celestial objects, 110, 190, 250, and sometimes one or two higher powers. The quantity of light it possesses is not much larger than that of the 3½ feet telescope, with 3¾ inches aperture; but the larger focal length of this telescope is considered to be an advantage; since the longer the focus of the object-glass, the less will be its chromatic and spherical aberrations, and the larger may be the eye-glasses, and the flatter the field of view.
The following are the prices of these telescopes as marked in Mr. Tulley’s catalogue.
| £ | s. | d. | |
|---|---|---|---|
| 5 feet telescopes 3¾ inches aperture, on an universal equatorial stand, with achromatic finder, 4 eye pieces for astronomical purposes, and 1 for day objects to vary the magnifying power, packed in a mahogany box 100 guineas to | 157 | 10 | 0 |
| 7 feet ditto, 5 inches aperture, on a newly improved universal equatorial stand, 6 eye pieces for astronomical purposes, and 1 for day objects to vary the magnifying power, with achromatic finder, and Troughton’s Micrometer | 207 | 5 | 0 |
The above are all the kinds of achromatic telescopes generally made by the London opticians. Those of the larger kind, as 5 and 7 feet telescopes, and the 3½ feet with 3¾ inches aperture, are generally made to order, and are not always to be procured. But the 2½ and 3½ feet achromatics of 2¾ inches aperture, are generally to be found ready-made at most of the optician’s shops in the metropolis. The prices of these instruments are nearly the same in most of the optician’s shops in London. Some of them demand a higher price, but few of them are ever sold lower than what has been stated above, unless in certain cases, where a discount is allowed.