Practical observations on telescopes.

  • William Kitchiner
Date:
1815
Catalogue details

Licence: Public Domain Mark

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

    89/116 page 87
    i 87 pands, that a greater quantity of light may enter the eye, and thus make a stronger impression upon it. This contraction and dilatation of the pupil may be observed by holding a looking-glass (or, what is still better, the lowest small speculum of a Gre¬ gorian telescope) before the eye at a win¬ dow, and turning gradually from it, con¬ tinually looking at the eye. It may be more easily and perfectly seen by atten¬ tively watching the eye of another. I think it is most agreeably observed in a fine full blue eye. To ascertain the magnifying power of a telescope, measure the diameter of the aper¬ ture of the object-glass, and that of the little image of it which is formed at the end of the eye-piece, the proportion between these will give the ratio of the magnifying power. To measure the diameter of the pencil of rays with great ease and accuracy, Mr. Ramsden*, about the year 1775, con- * The highest praise is due to the merits of the late Mr. Jesse Ramsden for his ingenuity, liberality, and persevering endeavours to invent and perfect the vari¬ ous instruments used in Astronomy, Philosophy, and
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