Page:Popular Science Monthly Volume 22.djvu/309

This page has been proofread, but needs to be validated.
295
THE GREAT COMET OF 1882.

great equatorial was very much what it had been on the 10th with the smaller telescope. There were no envelopes, and the only "jet" was the bright streak following the nucleus. The dark stripe had wholly disappeared, as if obliterated and replaced by the bright one. The "knots" in the nucleus were seen to be irregular in form, and were arranged not in a straight line, but in a somewhat broken curve, conforming to the curvature of the tail, which at this time extended 18°, and was fully 60,000,000 miles in length. The bright stream originated not at the extremity of the nucleus, but came out tangentially from the convex side, and perhaps had its source in the largest of the knots, which was now the third from the sunward extremity. The whole length of the nucleus measured 481/2", corresponding to a length of more than 40,000 miles, the diameter of the largest single mass being about 5,000 or 6,000 miles. The only other observation we have been able to make at Princeton was nine days later, on October 24th. No material changes were noticed, though the comet was very much fainter. The same lengthened granular nucleus continued, and seems likely to persist until the comet disappears.[1]

The spectroscopic observations have been very interesting. On September 18th the French physicist Thollon was an independent discoverer of the comet, coming upon it accidentally in sweeping around the sun. His spectroscopic apparatus consists of a so-called siderostat, the mirror of which throws the rays from the object to be examined upon the lens of a horizontal telescope nine and a half inches in diameter and about twenty feet long. At the focus of this telescope in a darkened room is placed a spectroscope, and, of course, this may be of any form and power best suited to the occasion. In the present case he used an instrument with a single prism of high dispersive power. The most marked feature of the spectrum was the presence of the lines of sodium in the spectrum of the nucleus. They were very bright, and were displaced toward the red by an amount equal to about one fourth of the interval between them, thus indicating that the comet was rapidly receding from the earth. A very narrow, bright, continuous spectrum was also shown by the nucleus. In this the dark lines of Fraunhofer were not conspicuous if visible at all,

  1. Later observations, on November 4th, show the same general characteristics. The nucleus, if it can be so called, was now 93" in length, or more than 90,000 miles. Three stellar points could be detected in the forward portion of the nucleus, but only two in the other. The separation between the two brightest points was about 10". The spectrum showed no new developments. To the naked eye the comet was unexpectedly bright, although now distant from both sun and earth nearly 140,000,000 miles. The head looked like a fourth-magnitude star, and the tail was 16° long and 4° wide at the extremity. On November 20th the nucleus had almost vanished, appearing merely as a brighter streak in the structureless nebulosity of the head. The tail was still nearly as large as ever, and easily visible without telescopic aid, though of course much fainter than on the 4th. The comet has held out remarkably, and, so far as now appears, it may be observable for a long time yet, especially in the southern hemisphere.