we find from their stellar magnitudes, taking the stellar magnitude found for Deimos by Pickering in 1877 as basis, their diameters to be,—
- Deimos, about 10 miles;
- Phobos, about 36 miles.
Phobos would thus, at its closest approach to the surface of the planet, that is, when it was in the zenith, just show a disk like the Moon. Otherwise both satellites would appear as stars.
Neither satellite shares the red tint of the planet.
As the means employed in any astronomical observation are of interest, I may add that the telescope used in these researches was an 18-inch refractor, made by Brashear, of Alleghany, Pa., the largest he has yet made. The powers used varied from 320 to 1305 diameters, the usual ones being, for visual purposes, 440 and 617, and, for micrometric measurements, 862. There is, not unnaturally, much misconception prevalent as to the magnification possible in a telescope. The highest powers of a glass can never be used on planetary detail, as the tremors of the air blur the image. Thus we come back again to the question of atmosphere, which is indeed the crux observationis. With regard to work on the planets, the important point about an observatory is not so much what is its lens as what is its location.