Now, if the observer's telescope is sufficiently powerful, each of the components may be seen to be itself double. First try ε1, the northern pair. The line joining the components is directed as shown in Plate [3]. The distance between them is 3"·2, their magnitudes 5 and 6½, and their colours yellow and ruddy. If the observer succeeds in seeing ε1 fairly divided, he will probably not fail in detecting the duplicity of ε2, though this is a rather closer pair, the distance between the components being only 2"·6. The magnitudes are 5 and 5½, both being white. Between ε1 and ε2 are three faint stars, possibly forming with the quadruple a single system.
Let us next turn to the third star of the equilateral triangle mentioned above—viz. to the star ζ Lyræ. This is a splendid but easy double. It is figured in Plate [3], but it must be noticed that the figure of ζ and the other nine small figures are not drawn on the same scale as ε Lyræ. The actual distance between the components of ζ Lyra is 44", or about one-fourth of the distance separating ε1 from ε2. The components of ζ are very nearly equal in magnitude, in colour topaz and green, the topaz component being estimated as of the fifth magnitude, the green component intermediate between the fifth and sixth magnitudes.
We may now turn to a star not figured in the map, but readily found. It will be noticed that the stars ε, α, β, and γ form, with two small stars towards the left, a somewhat regular hexagonal figure—a hexagon, in fact, having three equal long sides and three nearly equal short sides alternating with the others. The star η Lyræ forms the angle next to ε. It is a wide and unequal double, as figured in Plate [3]. The larger component is azure blue; the smaller is violet, and, being only of the ninth magnitude, is somewhat difficult to catch with apertures under 3 inches.
The star δ2 Lyræ is orange, δ1 blue. In the same field with these are seen many other stars.
The stars γ1 and γ2 may also be seen in a single field, the distance between them being about half the moon's mean diameter. The former is quadruple, the components being yellow, bluish, pale blue, and blue.
Turn next to the stars β and γ Lyræ, the former a multiple, the latter an unequal double star. It is not, however, in these respects that these stars are chiefly interesting, but for their variability. The variability of γ has not indeed been fully established, though it is certain that, having once been less bright, γ is now considerably brighter than β. The change, however, may be due to the variation of β alone. This star is one of the most remarkable variables known. Its period is 12d. 21h. 53m. 10s. In this time it passes from a maximum brilliancy—that of a star of the 3·4 magnitude—to a minimum lustre equal to that of a star of the 4·3 magnitude, thence to the same maximum brilliancy as before, thence to another minimum of lustre—that of a star of the 4·5 magnitude—and so to its maximum lustre again, when the cycle of changes recommences. These remarkable changes seem to point to the existence of two unequal dark satellites, whose dimensions bear a much greater proportion to those of the bright components of β Lyræ than the dimensions of the members of the Solar System bear to those of the sun. In this case, at any rate, the conjecture hazarded about Algol, that the star revolves around a dark central orb, would be insufficient to account for the observed variation.
Nearly midway between β and γ lies the wonderful ring-nebula 57 M, of which an imperfect idea will be conveyed by the last figure of Plate [3]. This nebula was discovered in 1772, by Darquier, at Toulouse. It is seen as a ring of light with very moderate telescopic power. In a good 3½-inch telescope the nebula exhibits a mottled appearance and a sparkling light. Larger instruments exhibit a faint light within the ring; and in Lord Rosse's great Telescope "wisps of stars" are seen within, and faint streaks of light stream from the outer border of the ring. This nebula has been subjected to spectrum-analysis by Mr. Huggins. It turns out to be a gaseous nebula! In fact, ring-nebulæ—of which only seven have been detected—seem to belong to the same class as the planetary nebulæ, all of which exhibit the line-spectrum indicative of gaseity. The brightest of the three lines seen in the spectrum of the ring-nebula in Lyra presents a rather peculiar appearance, "since it consists," says Mr. Huggins, "of two bright dots, corresponding to sections of the ring, and between these there is not darkness, but an excessively faint line joining them. This observation makes it probable that the faint nebulous matter occupying the central portion is similar in constitution to that of the ring."
The constellation Hercules also contains many very interesting objects. Let us first inspect a nebula presenting a remarkable contrast with that just described. I refer to the nebula 13 M, known as Halley's nebula (Plate [3]). This nebula is visible to the naked eye, and in a good telescope it is a most wonderful object: "perhaps no one ever saw it for the first time without uttering a shout of wonder." It requires a very powerful telescope completely to resolve this fine nebula, but the outlying streamers may be resolved with a good 3-inch telescope. Sir W. Herschel considered that the number of the stars composing this wonderful object was at least 14,000. The accepted views respecting nebulæ would place this and other clusters far beyond the limits of our sidereal system, and would make the component stars not very unequal (on the average) to our own sun. It seems to me far more probable, on the contrary, that the cluster belongs to our own system, and that its components are very much smaller than the average of separate stars. Perhaps the whole mass of the cluster does not exceed that of an average first-magnitude star.
The nebulæ 92 M and 50 H may be found, after a little searching, from the positions indicated in the map. They are both well worthy of study, the former being a very bright globular cluster, the latter a bright and large round nebula. The spectra of these, as of the great cluster, resemble the solar spectrum, being continuous, though, of course, very much fainter.
The star δ Herculis (seen at the bottom of the map) is a wide and easy double—a beautiful object. The components, situated as shown in Plate [3], are of the fourth and eighth magnitude, and coloured respectively greenish-white and grape-red.