Dr. See, observing with the large telescope of the Lowell Observatory, found that when the sky is clear, the moon absent, and the seeing perfect, “the sky appeared in patches to be of a brownish colour,” and suggests that this colour owes its existence to immense cosmical clouds, which are shining by excessively feeble light! Dr. See found that these brown patches seem to cluster in certain regions of the Milky Way.[371]
From a comparison of Trouvelot’s drawing of the small elongated nebula near the great nebula in Andromeda with recent photographs, Mr. Easton infers that this small nebula has probably rotated through an angle of about 15° in 25 years. An examination I have made of photographs taken in different years seems to me to confirm this suspicion, which, if true, is evidently a most interesting phenomenon.
Dr. Max Wolf of Heidelberg finds, by spectrum photography, that the well-known “ring nebula” in Lyra consists of four rings composed of four different gases. Calling the inner ring A, the next B, the next C, and the outer D, he finds that A is the smallest ring, and is composed of an unknown gas; the next largest, B, is composed of hydrogen gas; the next, C, consists of helium gas; and the outer and largest ring, D, is composed—like A—of an unknown gas. As the molecular weight of hydrogen is 2·016, and that of helium is 3·96, Prof. Bohuslav Brauner suggests that the molecular weight of the gas composing the inner ring A is smaller than that of hydrogen, and the molecular weight of the gas forming the outer ring D is greater than that of helium. He also suggests that the gas of ring A may possibly be identical with the “coronium” of the solar corona, for which Mendelief found a hypothetical atomic and molecular weight of 0·4.[372]
With reference to the nebular hypothesis of Laplace, Dr. A. R. Wallace argues that “if there exists a sun in a state of expansion in which our sun was when it extended to the orbit of Neptune, it would, even with a parallax of 1⁄60th of a second, show a disc of half a second, which could be seen with the Lick telescope.” My reply to this objection is, that with such an expansion there would probably be very little “intrinsic brightness,” and if luminous enough to be visible the spectrum would be that of a gaseous nebula, and no known star gives such a spectrum. But some planetary nebulæ look like small stars, and with high powers on large telescopes would probably show a disc. On these considerations, Dr. Wallace’s objection does not seem to be valid.
It is usually stated in popular works on astronomy that the spectra of gaseous nebulæ show only three or four bright lines on a faint continuous background. But this is quite incorrect. No less than forty bright lines have been seen and measured in the spectra of gaseous nebulæ.[373] This includes 2 lines of “nebulium,” 11 of hydrogen, 5 of helium, 1 of oxygen (?), 3 of nitrogen (?), 1 of silicon (?), and 17 of an unknown substance. In the great nebulæ in Orion 30 bright lines have been photographed.[374]
D’Arrest found that “gaseous nebulæ are rarely met with outside the Milky Way, and never at a considerable distance from it.”[375]
Mr. A. E. Fath thinks that “no spiral nebula investigated has a truly continuous spectrum.” He finds that so feeble is the intensity of the light of the spiral nebulæ that, while a spectrogram of Arcturus can be secured with the Mills spectrograph “in less than two minutes,” “an exposure of about 500 hours would be required for the great nebula in Andromeda, which is of the same spectral type.”[376] Mr. Fath thinks that in the case of the Andromeda nebula, the “star cluster” theory “seems to be the only one that can at all adequately explain the spectrum obtained.”[377]
Prof. Barnard finds that the great cluster in Hercules (Messier 13) is “composed of stars of different spectral types.” This result was confirmed by Mr. Fath.[378]
From observations with the great 40-inch telescope of the Yerkes Observatory (U.S.A.), Prof. Barnard finds that the nucleus of the planetary nebula H. iv. 18 in Andromeda is variable to the extent of at least 3 magnitudes. At its brightest it is about the 12th magnitude; and the period seems to be about 28 days. Barnard says, “I think this is the first case in which the nucleus of a planetary or other nebula has been shown to be certainly variable.” “The normal condition seems to be faint—the nucleus remaining bright for a few days only. In an ordinary telescope it looks like a small round disc of a bluish green colour.” He estimated the brightness of the nebula as that of a star of 8·2 magnitude.[379] Even in a telescope of 4 inches aperture, this would be a fairly bright object. It lies about 3½ degrees south-west of the star ι Andromedæ.
The so-called “globular clusters” usually include stars of different brightness; comparatively bright telescopic stars of the 10th to 13th magnitude with faint stars of the 15th to 17th magnitude. Prof. Perrine of the Lick Observatory finds that (a) “the division of the stars in globular clusters into groups, differing widely in brightness, is characteristic of these objects”; (b) “the globular clusters are devoid of true nebulosity”; and (c) “stars fainter than 15th magnitude predominate in the Milky Way and globular clusters, but elsewhere are relatively scarce.” He found that “exposures of one hour or thereabouts showed as many stars as exposures four to six times as long; the only effect of the longer exposures being in the matter of density.” This last result confirms the late Dr. Roberts’ conclusions. Perrine finds that for clusters in the Milky Way, the faint stars (15th to 17th magnitude) “are about as numerous in proportion to the bright stars (10th to 13th magnitude) as in the globular clusters themselves.” This is, however, not the case with globular clusters at a distance from the Milky Way. In these latter clusters he found that “in the regions outside the limits of the cluster there are usually very few faint stars, hardly more than one-fourth or one-tenth as many as there are bright stars”; and he thinks that “this paucity of faint stars” in the vicinity of these clusters “gives rise to the suspicion that all regions at a distance from the Galaxy may be almost devoid of these very faint stars.” The late Prof. Keeler’s series of nebular photographs “in or near the Milky Way” tend to confirm the above conclusions. Perrine finds the northernmost region of the Milky Way “to be almost, if not entirely, devoid of globular clusters.”[380]