Page:Philosophical Transactions - Volume 095.djvu/44

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Dr. Herschel's Experiments on the Means

of an object, and therefore had recourse to the following experiments.


1st Experiment, with the Heads of Pins.

I selected a set of pins with round heads, and deprived them of their polish by tarnishing them in the flame of a candle. The diameters of the heads were measured by a microscopic projection, with a magnifying power of 80. These measures are so exact, that when repeated they will seldom differ more than a few ten thousandths parts of an inch from each other. Their sizes were as follows: ,1375 ,0863 ,0821 ,0602 ,0425. I placed the pins in a regular order upon a small post erected in my garden, at 2407,85 inches from the centre of the object mirror of my ten-feet reflecting telescope. The focal length of the mirror on Arcturus is 119,64 inches, but on these objects 125,9. The distance was measured with deal rods.

When I looked at these objects in the telescope, I found immediately that only the smallest of them, at this distance could be of any use; for with an eye-glass of 4 inches, which gives the telescope a magnifying power of no more than 31,5, this pin's head appeared to be a round body, and the view left no doubt upon the subject. It subtended an angle of 3",64 at the centre of the mirror, and the magnified angle under which I saw it was 1' 54",6. This low power however required great attention.

With a lens 3,3, power 38,15, I saw it instantly round and globular. The magnified angle was 2' 18,9.

With a magnifying power of 231,8,[1] I saw it so plainly that

  1. The powers have been strictly ascertained as they are at the distance where these objects were viewed.