About the satellites Mars possesses, Deimos and Phobos, we may perhaps say a word, as recent knowledge concerning them exemplifies the care now taken to such ascertainment and the importance of considering factors often overlooked. Soon after they were discovered in 1877, they were measured photometrically, with the result of giving a diameter of six miles to Deimos and one of seven miles to Phobos, and these values unchallenged entered the text-books. When the satellites came to be critically considered at Flagstaff, it was found that these determinations were markedly in error, Phobos being very much the larger of the two, the actual values reaching nearer ten miles for Deimos and thirty-six for Phobos. In getting the Flagstaff values, the size to the eye of the satellite was corrected for the background upon which it shone; for the background is all-important to the brilliancy of a star. In the case of a small star near a planet, the swamping glare of the planet is something like the inverse cube of its distance away. Furthermore, the Flagstaff observations indicated how the previous error had crept in. For before correction for the differing brilliancies of the field of view, the apparent size of the satellites judged by conspicuousness was about six to seven. The photometric values must have been taken just as they came out, no correction apparently having been made for the background. Now the background is a fundamental factor in all photometric determinations, a factor somewhat too important in this case to neglect, since it affected the result 2500 per cent. CHAPTER IV THE OUTER PLANETS Beyond Mars lies the domain of the asteroids, a domain vast in extent, that, untenanted by any large planet, stretches out to Jupiter. Occupied solely by a host of little bodies agreeing only in lack of size, even this space seems too small to contain them, for recent research has shown some transgressing its bounds. One, Eros, discovered by De Witt, more than trenches on Mars’ territory, having an orbit smaller than that of the god of war, and may be considered perhaps the forerunner of more yet to be found between Mars and the Earth. On the other side, three recently detected by Max Wolf at Heidelberg have periods equal to that of Jupiter, and in their motions appear to exemplify an interesting case of celestial mechanics pointed out theoretically by Lagrange long before its corroboration in fact was so much as dreamt. Achilles, Patroclus, and Hector, as the triad are called, so move as always to keep their angular distance from Jupiter unaltered in their similar circuits of the Sun. [Illustration: ORBITS OF THE OUTER PLANETS.]