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The Practical Astronomer: Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomyDick, Thomas

Science

The Practical Astronomer: Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomy

Name: The Practical Astronomer: Comprising illustrations of light and colours--practical descriptions of all kinds of telescopes--the use of the equatorial-transit--circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomy
Author: Dick, Thomas

Dick, Thomas

Astronomical instruments; Astronomy; Telescopes

--------+-----------------------+-------------+------------------------
          |                       |             |
          |                       |  Radius of  |
          |                       |  both the   |
   Focal  |   Convex of crown     | surfaces of |    Convex lens of
  length. |         glass.        | the concave |      crown glass.
          |                       |  of flint   |
          |                       |    glass    |
  --------+-----------------------+-------------+------------------------
          |           |           |             |            |
  Inches. | Inc. Dec. | Inc. Dec. | Inc. Dec.   | Inc.  Dec. | Inc.  Dec.
      6   |   3.  77  |  4.   49  |  3.   47    |  3.    77  |  4.    49
      9   |   5.  65  |  6.   74  |  5.   21    |  5.    65  |  6.    74
     12   |   7.  54  |  8.   99  |  6.   95    |  7.    54  |  8.    99
     18   |  11.  30  | 13.   48  | 10.   42    | 11.    30  | 13.    48
     24   |  15.  08  | 17.   98  | 13.   90    | 15.    08  | 17.    98
     36   |  22.  61  | 26.   96  | 20.   84    | 22.    61  | 26.    96
     42   |  26.  38  | 31.   45  | 24.   31    | 26.    38  | 31.    45
     48   |  30.  16  | 35.   96  | 27.   80    | 30.    16  | 35.    96
     54   |  33.  91  | 40.   45  | 31.   27    | 33.    91  | 40.    45
     60   |  37.  68  | 44.   94  | 34.   74    | 37.    68  | 44.    94

From this table it appears, that the two convex lenses, have the same
radii of their respective sides and that the concave flint lens has
its two surfaces equally concave, so that a triple object-glass formed
according to these proportions, would require only three pair of
grinding tools. The following are the curves of the lenses of one of
the best of Dollond’s achromatic telescopes, the focal length of the
compound object-glass being 46 inches. Reckoning from the surface next
the object--the radii of the crown-glass were 28 and 40 inches: the
concave lens 20.9 inches, and the inner crown-glass lens, 28.4 and 28.4
inches. This telescope carried magnifying powers of from 100 to 200
times.

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