Page:Philosophical Transactions - Volume 095.djvu/45

This page has been proofread, but needs to be validated.
33
of ascertaining the Magnitude of small celestial Bodies.

the little notch in the pin's head between the coils of the wire making the head, appeared like a narrow black belt surrounding the pin in the manner of the belts of Jupiter. This notch by the microscopic projection measured ,00475 inch; and subtended an angle, at the centre of the mirror, of 0",407

With 303,5 I saw the belt still better, and could follow it easily in its contour.

With 432,0 I could see down into the notch, and saw it well defined within.

With 522,3 the pin's head was a very striking globular object, whose diameter might easily be divided by estimation into ten parts, each of which would be equal to 0",364.

With 925,6 I saw all the same phenomena still plainer.

The result of this experiment is, that an object having a diameter ,0425 may be easily seen in my telescope to be a round body, when the magnified angle under which it appears is 2' 18",9, and that with a high power a part of it, subtending an angle of 0",364 may be conveniently perceived.

When I considered the purpose of this experiment, I found the result not sufficient to answer my intention; for as the size of the object I viewed obliged me to use a low power, a doubt arose whether the instrument would be equally distinct when a higher should be required. To resolve this question, it was necessary either to remove my objects to a greater distance or to make them smaller.

 

2d Experiment, with small Globules of Sealing-wax.

I melted some sealing-wax thinly spread on a broad knife, and dipt the point of a fine needle, a little heated, into it, which took up a small globule. With some practice I soon acquired

MDCCCV.
F