The telescope and microscope / by Thomas Dick.

Thomas Dick

Date:: [1851?]

Credit: The telescope and microscope / by Thomas Dick. Source: Wellcome Collection.

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$two small reflecting telescopes, the construction of which somewhat differed from the plan Gregory had proposed in 1663. They were only 6 inches long, but were considered as equal in magnifying power to a common refracting telescope 6 feet in length. Nothing more was heard of reflecting telescopes till about half a century afterwards, when, in the year 1723, Mr. Hadley made a reflector on Newton’s plan, the large speculum of which was 62f inches focal distance, and 5 inches diameter, and was furnished with magnifying powers of from 190 to 230 times. It equalled in its performance the telescope of Huygens, 123 feet in length. Reflecting telescopes have been ever since in general use among astronomers, and have en- tirely superseded the long refractors of the seventeenth century. We shall give a brief description of each of the telescopes to which we have alluded. « 1. The Newtonian reflector.—This instrument is represented in fig. 16, where B a e f is the tube, in which is placed the concave speculum b E, which reflects the parallel rays o c, p d, to a plane speculum G, which is placed at half a right angle to the axis of the tube, as much$

The telescope and microscope / by Thomas Dick.

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