Page:Proceedings of the Royal Society of London Vol 1.djvu/189

Rh of the globules. For this purpose, he viewed it with apertures of diﬁerent dimensions and of diﬁerent kinds. Some of them being of the usual annular opening or outside rim, from 6'5 to 8'8 inches, which reﬂected less- than half the light of the mirror, produced a spu- rious disc less than '18 in diameter; while the whole light of the. mirror gave a disc of '31 : he thinks it fair to conclude, that it is not the quantity of light, but the part of the mirror from which it is re- ﬂected, that we are to look upon as the cause of the magnitude of the spurious discs of objects; and this. he says, points out an im- proved method of putting any terrestrial disc, that we suspect to be spurious, to the test : for the inside rays of a mirrorwiil increase the' diameter of those discs; but the outside rays alone will have agreater eﬂect in reducing it, than when the inside rays are left to join with them.

Dr. Herschel then placed two other globules at a small distance from each other, without having previously measured either their size or the distance between them. Upon viewing them with a power of 522'7, they appeared in the shape of half-moons; he estimated the vacancy between the cusps to be one fourth the diameter of the largest; and found afterwards, on measuring the diameters and di- stance, that his estimation did not diﬁ'er Thth of an inch from the’ truth. In a second experiment the difference between the real and the estimated distance was still less.

In order to ascertain whether these half-moons were real or spu- rious, Dr. Herschel viewed them ﬁrst with the inside rays of the mirror, then with the outside rays, and lastly, with the whole mirror open, but no alteration in the distance of the lunes could be per- ceived. He then divided the aperture of the mirror into two parts, one from 0 to 4‘4 inches, the other from 4'4 to 8‘8; and found, on measuring the spurious diameter of a globule, that with the inside rays it was '40; with the whole mirror open it was '31 ; and with the outside rays it was '22.

From this, he says, we may conclude, that the diameters given by the inside rays, by all the mirror open, and by the outside rays, are in an arithmetical progression; and that the inside rays will nearly double the diameter given by the outside.

a Lyrae being then examined in the same manner, its spurious disc was found to be small with the outside rays; with the whole mirror open it was larger; and with the inside rays it was largest.

The double star a Geminorum was then viewed with a power of 410-5 ; with the outside rays they appeared unequal, and 1% diameter of the largest asunder; with the whole mirror open they were more unequal, and 1% diameter of the largest asunder; with the inside rays they were very unequal, and 1% of the largest asunder.

The foregoing experiments show, the author says, that if it had not been known that the apparent discs of the stars were spurious, the application of the improved criterion of the aperture would have discovered them to be so; and that, consequently, the same improvement is perfectly applicable to celestial objects.