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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    objects that are uncommonly well lighted up, the extent of vision being limited by the myriads of heterogeneous particles that are constantly floating in the air : these, by their opacity and reflective power, form a kind of veil that obscures the vision of remote objects : and the more the medium is loaded with these particles, and the more distant the object, the more obscure and indistinct it will appear. So, for determining the distinctness of our telescopes, we try them at objects not more than a few hundred feet distant from us. The exhalations which continually rise from the earth, aug¬ ment this impediment, and render the air less transparent, especially near the earth. But the obscurity arising from the exhala¬ tions, is not the least part of the incon¬ venience they occasion ; they have a kind of undulating motion, like that of smoke or steam, so that objects seen through them appear to have a tremulous, or dancing motion, which is sometimes sensible even to the naked eye. If distant objects be viewed on a hot summer’s day, this effect is
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