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This instrument, 150 feet in length, with a skeleton tube, was constructed by the celebrated seventeenth century astronomer, Hevelius of Danzig. From an illustration in the Machina Celestis.

The attempts to construct large telescopes of the Galilean type met in course of time with a great difficulty. The magnified image of the object observed was not quite pure; its edges, indeed, were fringed with rainbow-like colours. This defect was found to be aggravated with increase in the size of object-glasses. A method was, however, discovered of diminishing this colouration, or chromatic aberration as it is called from the Greek word χρῶμα (chroma), which means colour, viz. by making telescopes of great length and only a few inches in width. But the remedy was, in a way, worse than the disease; for telescopes thus became of such huge proportions as to be too unwieldy for use. Attempts were made to evade this unwieldiness by constructing them with skeleton tubes.

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The Project Gutenberg EBook of Astronomy of To-day, by Cecil G. Dolmage Published 1910

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Friday 17 April 2020

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