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Updated: June 10, 2025
To put it in another way: Jupiter, at the distance of the nearest stars, would be not far from one hundred times less bright than the faintest star which the largest telescope is just able, under the most exquisite conditions, to glimpse. To see a star so faint as that would require an object-glass of a diameter half as great as the length of the tube of the Lick telescope, or say thirty feet!
Our old friend Time has this year illustrated his march, or object-glass, with a host of images or spectra that is, woodcuts of head and tail pieces to suit all tastes from the mouldering cloister of other days to the last balloon ascent.
Any of the various fluorescing substances placed in the focus of the object-glass, or at the optical image in front of the eyepiece, will show the picture in the color peculiar to the fluorescing material. The color does not matter." "More simple still," laughed the admiral. "But how about the colored lights they saw?" "Simply the discarded light of the spectrum.
She could manage this glass to the best advantage, through her father's teaching, and could take out the slide and clean the lenses, and even part the object-glass, and refix it as well as possible. She belonged to the order of the clever virgins, but scarcely to that of the wise ones.
Now, as the curvature of the object-glass was known, it was easy to calculate the thickness of the plate of air at which any particular colour appeared, and thus to determine the law of the phenomena. By accurate measurements Newton found that the thickness of air at which the most luminous parts of the first rings were produced were, in parts of an inch, as 1, 3, 5, 7, 9, and 11 to 178,000.
Nearly every one interested in the subject is aware that this object-glass is composed of two lenses a concave one of flint-glass and a convex one of crown-glass, the latter being on the side towards the object. This is the one vital part of the telescope, the construction of which involves the greatest difficulty.
My father possessed a very excellent achromatic spy-glass of 2 inches diameter. The object-glass was made by the celebrated Ramsden. When I was about fifteen I used it to gaze at the moon, planets, and sun-spots.
It is interesting to adjust the telescope, and bring the starry system nearer to the vision. If we direct our gaze upon a planet, we find its disk or face sharply defined; change the direction, and let the object-glass rest upon a star, and we have only a point of light more or less brilliant.
Upon this principle he founded in 1817 his method of "limiting apertures," by which two stars, brought into view in two precisely similar telescopes, were "equalised" by covering a certain portion of the object-glass collecting the more brilliant rays.
The first, constructed by Herschel, was thirty-six feet in length, and had an object-glass of four feet six inches; it possessed a magnifying power of 6,000. The second was raised in Ireland, in Parsonstown Park, and belongs to Lord Rosse.
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