_Satellites of Mars._—After evading the keen and searching eyes of Sir W. Herschel, and the power of his 40-foot telescope—after eluding the grasp of Lord Rosse’s 6-foot speculum, and the frequent scrutiny of Lassell with his 2-and 4-foot mirrors, the two satellites of Mars were ultimately revealed to Prof. Hall in the 25·8-inch refractor at Washington. These tiny orbs had been enabled to avoid previous discovery by their minuteness and by their close proximity to Mars. Yet as soon as they were known to exist many observers saw them, and in certain cases success was undoubtedly attained with comparatively small instruments. The late Dr. Erck picked up the outermost satellite with a 7-1/3-inch objective, and Mr. Pratt saw it with an 8-1/7-inch mirror by With. But the effect of this eye-straining may just possibly, in one or two instances, have drawn the imagination out of its normal repose. Mr. Pratt’s instrument shows stars in the group ε Lyræ which are invisible in the great Washington telescope and in the 36-inch mirror formerly used by Mr. Common; so that it may well have produced a spectral satellite of Mars. But the satellites are certainly within the occasional reach of moderate means; for they were repeatedly seen with a 9½-inch refractor at the Observatory of Princeton, U.S.A., in October and November 1879. They “were decidedly more easy to see than Mimas,” the innermost satellite of Saturn. Phobos, the inner satellite, revolves round the planet in 7^h 39^m, in an orbit 6000 miles from the centre of Mars. At max. elongation the satellite is about 12″ distant from its primary, and its opposition magnitude is 11½. Deimos, the outer satellite, revolves in 30^h 18^m, and its orbit is 15,000 miles distant from Mars. Its elongations extend to 32″, and its opposition mag. is 13½. These diminutive objects are probably not more than 10 miles in diameter. They are obviously too faint for common instruments, nor are they objects on which ordinary amateurs may occupy themselves with advantage. Of course it forms a highly interesting spectacle to glimpse, just for once, it may be, the small bodies which resisted telescopic power for more than two and a half centuries; but for really useful observations, large aperture and means of accurate measurement are required.