2. _The 3-1/2 feet Achromatic Telescope._--The object glass of this telescope is from 44 to 46 inches focal length, and 2-3/4 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 2-1/2 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 3-1/2 feet achromatic telescope 2-3/4 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. £ s. d. The 3-1/2 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