Practical observations on telescopes.

  • William Kitchiner
Date:
1815
Catalogue details

Licence: Public Domain Mark

Credit: Practical observations on telescopes. Source: Wellcome Collection.

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    into 15, 10, towards 0, and 5 on the con¬ trary side. The revolutions of the micrometer head will bring the edge of the circle round it, and the division on the nonius, to coin¬ cide at 10: each division, therefore, is equal to the ten thousandth part of an inch. Applying this little instrument to the eye-glass of a telescope, when adjusted to distinct vision at any distant object, and turning the micrometer head, the emergent pencil will begin to separate; and when the extreme edges are brought into contact, the number of divisions will show the diameter of it in thousandths of an inch; then reduce the diameter of the object-glass into thousands, and divide that sum by the diameter of the pencil, the quotient will be the real magnifying power. But as it is requisite for the emergent pencil of rays to be in the focus of the divided glass, a thin transparent piece of ivory, precisely one- tenth of an inch in diameter, is set in the sliding cover, to adjust for that distance.
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