Page:Philosophical Transactions - Volume 095.djvu/47

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of ascertaining the Magnitude of small celestial Bodies.

power, will show the disk of a faint object when the angle it subtends at the naked eye is no more than 0",653.


3d Experiment, with Globules of Silver.

As the objects made of sealing-wax, on account of their colour, did not appear to be fairly selected for these investigations, I made a set of silver ones. They were formed by running the end of silver wires, the 305th and 340th part of an inch in diameter, into the flame of a candle. It requires some practice to get them globular, as they are very apt to assume the shape of a pear; but they are so easily made that we have only to reject those which do not succeed.

Thirteen of them, in a pretty regular succession of magnitude, were selected and placed upon the post. Their dimensions were ,03956 ,0371 ,0329 ,0317 ,0272 ,0260 ,0187 ,0178 ,0164 ,0125 ,01137 ,00800 ,00556.

For the sake of more conveniency I had removed my telescope from its station in the library to a work-room. The distance of the objects from the mirror of the telescope, measured with deal rods, was here only 2370,5 inches; and the focal length of the mirror, the magnifying powers of the telescope, and the angles subtended by the objects have been calculated accordingly.

With 522,7 I see all the globules, from No. 1 to No. 13, perfectly well, and can estimate the latter in quarters of its diameter. The angle it subtends at the centre of the mirror is 0",484; and one quarter of it is 0",121.

With the same power I see the wires which hold the balls, so well that even the smallest of them may be divided into

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