Especially significant is the fact that Venus with the Mexicans played as important a part as did the Moon and the Sun. The luminosity of Venus, unlike that of planets outside of the earth’s orbit, but like that of the Moon, changes from a maximum, intense enough in the tropics to throw a shadow, down to a minimum which approaches complete darkness. Its period is closely 1.6 years. It falls short of this figure by about two hours and the Mexicans therefore introduced a correction similar to the bissextile day in our leap year, of one day in every twelve years, which day, however, had to be deducted instead of added. Observations of the changing luminosity of Venus and of its position relative to the Sun obviously lent themselves admirably to measurements of long periods of time and particularly to determination of the length of the important solar year as five Venus-periods very nearly equal eight solar years. The Mexican priests established the fact that 104 solar years correspond to 65 Venus-periods or 146 “_Tonalamatl_.” Star-cult was as strongly developed in Mexico as in Babylon. Its main doctrine is stated by Alfredo Chavero thus: “The Father-Creator was Heaven, Xiuhtecutli, or the Azure-blue master. The mother was Omecihuatl, the Milky Way, or the dual mistress.” It is well known that a large part of the Milky Way, from the “Swan” to the neighbourhood of the Southern Cross, is divided in two parallel branches, which fact probably is responsible for the title, the “Dual Mistress.” “Heaven influenced the Milky Way through fire; from its cosmic matter the stars were set free, the most prominent of which were Tonatiuh, the Sun; Tezcatlipoca, the Moon; and Quetzalcoatl, Venus. These were made the supreme gods. For the purpose of worship, they were symbolized in human shape. Myriads of images, representing these star-gods, were modelled in clay, wood, or stone.” According to this remarkable picture the Mexicans should have arrived at a far better solution of the world-riddle than even the Babylonians did. While most other peoples assumed heaven and earth as the original principles, they gave the high position of progenitress of all to the Milky Way. From her the innumerable stars, with the Sun in the lead, issued. This agrees to a considerable extent with the present conception which we have arrived at during this very last decade, principally thanks to the work of American astronomers. Their investigations have shown how the stars are segregated from the nebulous primeval matter of the Milky Way; how they add distance between themselves and their matrix with age, while simultaneously they develop an increasing individuality.