according to Dr. Kitchener. ---------------------------------------------------+ ACHROMATIC REFRACTORS. | -----------+-----------+--------------+------------+ Length | Diameter | | | and name | of | Illuminating | Price. | they are | aperture. | power. | | called by. | | | | -----------+-----------+--------------+------------+ Feet. | In. Th. | | £ s. | 2 | 1. 6 | 25 | 4 4 | 2-1/2 | 2 | 40 | 12 12 | 3-1/2 | 2. 7 | 72 | 21 to 42 | 5 | 3. 8 | 144 | 105 to 150 | 7 | 5 | 250 | 250 | 7 | 6 | 360 | 360 | ---------------------------------------------------- | GREGORIAN &c. REFLECTORS. +--------------+-----------+--------------+--------- | Length and | Diameter | | | name they | of | Illuminating | Price. | are | aperture. | power. | | known by. | | | +--------------+-----------+--------------+--------- | Feet. | In. Th. | | £ s. | 1 | 2. 5 | 62 | 7 7 | 1-1/2 | 3 | 90 | 12 12 | 2 | 4. 5 | 202 | 20 | 3 | 5. 5 | 302 | 50 | 4 | 7 | 490 | 105 | 7 Newtonian | 7 | 490 | 126 | 5 Gregorian | 9 | 810 | 200 | 10 Newtonian | 10 | 1000 | 315 The illuminating powers stated in the above table are only comparative. Fixing on the number 25 as the illuminating power of a 2 feet telescope, 1-6/10 inch aperture, that of a 2-1/2 feet 2 inches inches aperture, will be 40, of a 5 feet 3-8/10 inch aperture, 144, &c. If the illuminating power of a Gregorian 1-1/2 foot, and 3 inches aperture, be 90, a 5 feet, with 9 inches aperture, will be 810, &c. 6. _On choosing Telescopes, and ascertaining their properties._ It is an object of considerable importance, to every astronomical observer, that he should be enabled to form a judgment of the qualities of his telescope, and of any instruments of this description which he may intend to purchase. The following directions may perhaps be useful to the reader in directing him in the choice of an achromatic refracting telescope.