A: Focal distance of the great mirror. B: Breadth of the great mirror. C: Focus of the small speculum. D: Breadth of the hole in the great speculum. E: Distance between the small speculum and the first eye-glass. F: Focal distance of the glass next the metals. G: Focal distance of the glass next the eye. H: Distance between the plain sides of the two glasses. I: Magnifying power. J: Distance between the second glass and the small eye-hole. --------+--------+--------+--------+-------- | | | | A. | B. | C. | D. | E. --------+--------+--------+--------+-------- | | | | P _m_ | D F | L _n_ | U V | L R | | | | In. Dec.|In. Dec.|In. Dec.|In. Dec.|In. Dec. | | | | 5. 65 | 1. 54 | 1. 10 | 0. 31 | 8. 54 9. 60 | 2. 30 | 1. 50 | 0. 39 |14. 61 15. 50 | 3. 30 | 2. 14 | 0. 50 |23. 81 36. 00 | 6. 26 | 3. 43 | 0. 65 |41. 16 60. 00 | 9. 21 | 5. 00 | 0. 85 |68. 17 --------+--------+---------+-----+------ | | | | F. | G. | H. | I. | J. --------+--------+---------+-----+------ | | | | R | S | R S | | | | | | In. Dec.|In. Dec.|In. Dec. | In. | | | | | 2. 44 | 0. 81 | 1. 68 | 39 | 0. 41 3. 13 | 1. 04 | 2. 09 | 60 | 0. 52 3. 94 | 1. 31 | 2. 63 | 86 | 0. 66 5. 12 | 1. 71 | 3. 41 | 165 | 0. 85 6. 43 | 2. 14 | 4. 28 | 243 | 1. 07 Mr. Short--who was born in Edinburgh in 1710, and died near London, 1768--was considered as the most accurate constructor of reflecting telescopes, during the period which intervened from 1732, to 1768. In 1743, he constructed a reflector for Lord Thomas Spencer, of 12 feet focal length, for which he received 600 guineas. He made several other telescopes of the same focal distance, with greater improvements and higher magnifiers; and in 1752, finished one for the king of Spain, for which, with its whole apparatus, he received £1200. This was considered the noblest instrument of its kind that had then been constructed, and perhaps it was never surpassed, till Herschel constructed his twenty and forty feet reflectors. High as the prices of large telescopes now are, Mr. Short charged for his instruments at a much higher rate than opticians now do, although the price of labour, and every other article required in the construction of a telescope, is now much dearer. But he had then scarcely any competitor, and he spared neither trouble nor expense to make his telescopes perfect, and put such a price upon them as properly repaid him. The following table contains a statement of the apertures, powers, and prices of Gregorian telescopes, as constructed by Mr. James Short.[26]