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 nebulae 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 nebulae 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 [delta] 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. The star [kappa] Herculis is not shown in the map, but may be very readily found, lying between the two gammas, [gamma] Herculis and [gamma] Serpentis (_see_ Frontispiece, Map 2), rather nearer the latter. It is a wide double, the components of fifth and seventh magnitude, the larger yellowish-white, the smaller ruddy yellow.[5] Ras Algethi, or [alpha] Herculis, is also beyond the limits of the map, but may be easily found by means of Map 2, Frontispiece. It is, properly speaking, a multiple star. Considered as a double, the arrangement of the components is that shown in Plate 3. The larger is of magnitude 3-1/2, the smaller of magnitude 5-1/2; the former orange, the latter emerald. The companion stars are small, and require a good telescope to be well seen. Ras Algethi is a variable, changing from magnitude 3 to magnitude 3-1/2 in a period of 66-1/3 days. The star [rho] Herculis is a closer double. The components are 3".7 apart, and situated as shown in Plate 3. The larger is of magnitude 4, the smaller 5-1/2; the former bluish-white, the latter pale emerald.