The westward track on high was determined by the early astronomers of Egypt. Thales, the father of Greek astronomy, made great advances upon the speculations he derived from the Egyptians, and expounded them in his own country. A scholar of his was the first person who pointed out the obliquity of the circle in which the sun moves among the stars, and thus "opened the gate of nature." Certainly he who had a clear view of that path in the celestial sphere, made that first step which led to all the rest. But when Greek science fell with Ptolemy, there was apparently no further advance till the rise of Copernicus. During this interval of thirteen hundred and fifty years, as before stated, the principal cultivators of astronomical science were the Arabians, who won their attainments from the Greeks whom they conquered, and from whom the conquerors of western Europe again received back their treasure when the love of science and the capacity for its use had been sufficiently awakened in their minds. In mechanics, also, no marked advancement was made from Archimedes till the time of Galileo and Stevinus. The same was true of hydrostatics, the fundamental problems of which were solved by the same great teacher, whose principles remained unpursued till the age of Leo X. began to give perfection to the true Archimedean form of science. As early as Euclid, mathematicians drew their conclusions respecting light and vision by the aid of geometry; as, for instance, the convergence of rays which fall on a concave speculum. But, down to a late period, the learned maintained that seeing is exercised by rays proceeding from the eye, not to it; so little was the real truth of optical science understood. In this respect, as in most others, it was attempted to explain the kind of causation in which scientific action originates, rather than to define the laws by which the process is controlled.