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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

The following table contains a statement of the apertures and
magnifying powers of Newtonian Telescopes, and the focal distances of
their eye-glasses. The first column contains the focal length of the
great speculum in feet; the second, its linear aperture in inches;
the third, the focal distance of the single glass in decimals, or in
1000ths of an inch, and the fourth column, contains the magnifying
power. This portion of the table was constructed by using the
dimensions of Mr. Hadley’s Newtonian Telescope, formerly referred to,
as a standard--the focal distance of the great mirror being 62-1/2
inches, its medium aperture 5 inches, and power 208. The fifth, sixth,
and seventh columns contains the apertures of the concave speculum,
the focal lengths of the eye-glasses and the magnifying powers, as
calculated by Sir D. Brewster, from a telescope of Mr. Hauksbee, taken
as a standard; whose focal length was 3 feet 3 inches, its aperture
about 4 inches, and magnifying power 226 times.

  A: Focal distance  of concave metal.
  B: Aperture of concave metal.
  C: Focal distance of single eye-glass.
  D: Magnifying power.
  E: Aperture of the concave speculum.
  F: Focal length of the eye-glass.
  G: Magnifying power.
  --------+----------+----------++----------------------------------
          |          |          ||
          |          |          ||    Sir D. Brewster’s Numbers.
     A.   |    B.    |    C.    ++----------------------------------
          |          |          ||     |          |            |
          |          |          ||  D. |    E.    |     F.     | G.
  --------+----------+----------++-----+----------+------------+----
          |          |          ||     |          |            |
    Feet. | In. Dec. | In. Dec. ||     | In. Dec. | In. Dec.   |
    0-1/2 |  0.  86  | 0.  167  ||  36 |  1.  34  | 0.  107    |  56
    1     |  1.  44  | 0.  199  ||  60 |  2.  23  | 0.  129    |  93
    2     |  2.  45  | 0.  236  || 102 |  3.  79  | 0.  152    | 158
    3     |  3.  31  | 0.  261  || 138 |  5.  14  | 0.  168    | 214
    4     |  4.  10  | 0.  281  || 171 |  6.  36  | 0.  181    | 265
    5     |  4.  85  | 0.  297  || 202 |  7.  51  | 0.  192    | 313
    6     |  5.  57  | 0.  311  || 232 |  8.  64  | 0.  200=1/5| 360
    7     |  6.  24  | 0.  323  || 260 |  9.  67  | 0.  209    | 403
    8     |  6.  89  | 0.  334  || 287 | 10.  44  | 0.  218    | 445
    9     |  7.  54  | 0.  344  || 314 | 11.  69  | 0.  222    | 487
   10     |  8.  16  | 0.  353  || 340 | 12.  65  | 0.  228    | 527
   11     |  8.  76  | 0.  362  || 365 | 13.  58  | 0.  233    | 566
   12     |  9.  36  | 0.  367  || 390 | 14.  50  | 0.  238    | 604
   13     |  9.  94  | 0.  377  || 414 | 15.  41  | 0.  243    | 642
   14     | 10.  49  | 0.  384  || 437 | 16.  25  | 0.  248    | 677
   15     | 11.  04  | 0.  391  || 460 | 17.  11  | 0.  252    | 713
   16     | 11.  59  | 0.  397  || 483 | 17.  98  | 0.  256    | 749
   17     | 12.

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