When the aperture of the object glass of this telescope exceeds 2-3/4 inches its price rapidly advances.
The following is Mr. Tulley’s scale of prices, proportionate to the increase of aperture:--
£ s. d. 3-1/2 feet telescopes 3-1/4 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, 3-3/4 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 2-3/4 inches aperture, is as follows:--The square of the 2-3/4 object-glass is 7.56; that of 3-1/4, 10.56, and that of the 3-3/4, 14.06; so that the light admitted by the 3-1/4 compared with the 2-3/4 aperture, is nearly as 10 to 7; and the light admitted by the 3-3/4 object-glass is nearly double that of the 2-3/4 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 3-8/10 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 3-1/2 feet telescope, with 3-3/4 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 3-3/4 inches aperture, on an universal equatorial stand, with achromatic finder, 4 eye pieces for astronomical purposes, and 1 for day objects to vary the power, packed in a mahogany box 100 guineas to 157 10 0