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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

As some ingenious mechanics may feel a desire to attempt the
construction of a compound achromatic object-glass, I shall here state
some of the proportions of curvature of the concave and convex lenses,
which serve to guide opticians in their construction of achromatic
instruments. These proportions are various; and even when demonstrated
to be mathematically correct, it is sometimes difficult to reduce them
to practice, on account of the different powers of refraction and
dispersion possessed by different discs of crown and flint-glass, and
of the difficulty of producing by mechanical means, the exact curves
which theory requires. The following table shows the radii of curvature
of the different surfaces of the lenses necessary to form _a double
achromatic object-glass_--it being supposed that the sine of refraction
in the crown-glass is as 1.528 to 1, and in the flint as 1.5735 to 1;
the ratio of their dispersive powers being as 1 to 1.524. It is also
assumed that the curvatures of the concave lens are as 1 to 2, that
is, that the one side of this lens is ground on a tool, the radius
of which is double that of the other. The 1st column expresses the
compound focus of the object-glass in _inches_; the 2nd column states
the radius of the _anterior_ surface of the _crown_, and column 3rd,
its _posterior_ side. Column 4th expresses the radius of the anterior
surface of the _concave_ lens, and column 5th its posterior surface,
which, it will be observed, is exactly double that of the other.

  -------+-----------+-----------+-----------+-----------
  Focus  |Radius of  |Radius of  |Radius of  |Radius of
  in     |anterior   |posterior  |anterior   |posterior
  inches.|surface,   |surface.   |surface,   |surface.
         |convex.    |           |concave.   |
  -------+-----------+-----------+-----------+-----------
         |Inc.  Dec. |Inc.  Dec. |Inc.  Dec. |Inc.  Dec.
    12   |  3        |  4.   652 |  4.   171 |  8.   342
    24   |  6        |  9.   304 |  8.   342 | 16.   684
    30   |  7.    5  | 11.   063 | 10.   428 | 20.   856
    36   |  9        | 13.   956 | 12.   513 | 25.   027
    48   | 12        | 18.   608 | 16.   684 | 33.   369
    60   | 15        | 23.   260 | 20.   856 | 41.   712
   120   | 30        | 46.   520 | 41.   712 | 83.   424

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