The _Double Stars_ require a great _variety_ of powers, in order to distinguish the small stars that accompany the larger. Some of them are distinguished with moderate powers, while others require pretty large instruments, furnished with high magnifying eye-pieces. I shall therefore select only a few as a specimen. The star _Castor_, or α Geminorum, may be easily seen to be double with powers of from 70 to 100. I have sometimes seen these stars, which are nearly equal in size and colour, with a terrestrial power of 44 on a 44-inch achromatic. The appearance of this star with such powers is somewhat similar to that of η Coronæ in a 7 feet achromatic, of 5 inches aperture, with a power of 500. γ Andromedæ may be seen with a moderate power. In a 30-inch achromatic of 2 inches aperture, and a power of 80, it appears like ε Bootis, when seen in a 5-feet achromatic, with a power of 460. This star is said to be visible even in a 1-foot achromatic with a power of 35. ε _Lyræ_, which is a quintuple star, but appears to the naked eye as a single star,--may be seen to be double with a power of from 6 to 12 time. γ _Leonis_ is visible in a 44-inch achromatic, with a power of 180 or 200. _Rigel_ in a 3-1/2-feet achromatic, may be seen with powers varying from 130 to 200. The small star, however, which accompanies Rigel, is sometimes difficult to be perceived, even with such powers. ε _Bootis_ is seldom distinctly defined with an achromatic of less aperture than 3-1/4 inches, or a reflector of less than 5 inches, with a power of at least 250. These and similar stars are not to be expected to be seen equally well at all times, even when the magnifying and illuminating powers are properly proportioned; as much depends upon the state of the weather, and the pureness of the atmosphere. In order to perceive the closest of the double stars, Sir W. Herschel recommends, that the power of the telescope should be adjusted upon a star known to be single, of nearly the same altitude, magnitude, and colour with the double star which is to be observed, or upon one star above and another below it. Thus, the late Mr. Aubert, the astronomer, could not see the two stars of γ Leonis, when the focus was adjusted upon that star itself; but he soon observed the small star, after he had adjusted the focus upon Regulus. An exact adjustment of the focus of the instrument is indispensably requisite, in order to perceive such minute objects.