Sweeping northwestward to ζ, we find a celebrated binary, to separate which will require the higher powers of our five inch glass. The magnitudes are three and six and a half, distance in 1894 1·28″, p. 40°. The period of revolution is thirty-five years, and two complete revolutions have been observed. The apparent distance changes from 0⋅6″ to 1·6″. They were at their extreme distance in 1884 and are now closing.
Two pleasing little doubles are Σ 2101, magnitudes six and nine, distance 4″, p. 57°, and Σ 2104, magnitudes six and eight, distance 6″, p. 20°. At the northern end of the constellation is 42, a double that requires the light-grasping power of our largest glass. Its magnitudes are six and twelve, distance 20″, p. 94°. In ρ, we discover another distinctly colored double, both stars being greenish or bluish, with a difference of tone. The magnitudes are four and five and a half, distance 37″, p. 309°. But the double 95 is yet more remarkable for the colors of its stars. Their magnitudes are five and five and a half, distance 6″, p. 262°, colors, according to Webb, "light apple-green and cherry-red." But other observers have noted different hues, one calling them both golden yellow. I think Webb's description is more nearly correct. Σ 2215 is a very close double, requiring larger telescopes than those we are working with. Its magnitudes are six and a half and eight, distance 0·7″, p. 300°. It is probably a binary. Σ 2289 is also close, but our five-inch will separate it: magnitudes six and seven, distance 1⋅2″, p. 230°.
Turning to μ, we have to deal with a triple, one of whose stars is at present beyond the reach of our instruments. The magnitudes of the two that we see are four and ten, distance 31″, p. 243°. The tenth-magnitude star is a binary of short period (probably less than fifty years), the distance of whose components was 2″ in 1859, 1″ in 1880, 0·34″ in 1889, and 0·54″ in 1891, when the position angle was 25°, and rapidly increasing. The distance is still much less than 1″.
For a glance at a planetary nebula we may turn with the five-inch to No. 4234. It is very small and faint, only 8″ in diameter, and equal in brightness to an eighth-magnitude star. Only close gazing shows that it is not sharply defined like a star, and that it possesses a bluish tint. Its spectrum is gaseous.
The chief attraction of Hercules we have left for the last, the famous star cluster between η, and ζ, No. 4230, more commonly known as M 13. On a still evening in the early summer, when the moon is absent and the quiet that the earth enjoys seems an influence descending from the brooding stars, the spectacle of this sun cluster in Hercules, viewed with a telescope of not less than five-inches aperture, captivates the mind of the most uncontemplative observer. With the Lick telescope I have watched it
