Page:Philosophical Transactions - Volume 095.djvu/64

This page has been proofread, but needs to be validated.
Dr. Herschel's Experiments on the Means

Every thing being now arranged for the measurement, I viewed the spurious diameter, with a magnifying power of 522,7, and compared it to the circles which succeeded each other by small differences of magnitude.

With all the mirror, from the centre to 8,8 inches open, the diameter of the spurious disk was ,31 inches.

With 6,3 inches open, it was less than ,40 and larger than ,355.

With 5 inches open, it was ,40.

With 4 inches open, it was ,42.

With 3 inches open, it was ,465 nearly.

From these measures it might be supposed that by lessening the quantity of light, we bring on a certain indistinctness which gives more diameter to the spurious object; to prove that this is not the cause of the increase, I used the following apertures.

With an annular opening from 6,5 to 8,8 inches, the spurious disk was rather less than ,18.

With another from 5 to 8,8 it was exactly ,18.

With an opening from 4 to 6,5 it was ,22.

With another from 1,6 to 4 it was ,42.

(12.) Now since the outside rim from 6,5 to 8,8, which reflected less than half the light of the mirror, produced a spurious disk less than ,18 in diameter, and the whole light as we have seen gave a disk of ,31, it is evidently not the quantity of the light, but the part of the mirror from which it is reflected, that we are to look upon as the cause of the magnitude of the spurious disks of objects.

(13.) These measures therefore point out an improvement in my former method of putting any terrestrial disk we suspect to be spurious to the test. For the inside rays of a mirror, as