Roger Bacon, an Oxford master, began the science of physics. He read Arab writers on the source of light rays being from the object seen, the nature of refraction and reflection of light, and the properties of lenses. He studied the radiation of light and heat. He studied angles of reflection in plane, spherical, cylindrical, and conical mirrors, in both their concave and convex aspects. He did experiments in refraction in different media, e.g. air, water, and glass, and knew that the human cornea refracted light, and that the human eye lens was doubly convex. He comprehended the magnifying power of convex lenses and conceptualized the combination of lenses which would increase the power of vision by magnification. He realized that rays of light pass so much faster than those of sound or smell that the time is imperceptible to humans. He knew that rays of heat and sound penetrate all matter without our awareness and that opaque bodies offered resistance to passage of light rays. He knew the power of parabolic concave mirrors to cause parallel rays to converge after reflection to a focus and knew that a mirror could be produced that would start a fire at a fixed distance. These insights made it possible for jewelers and weavers to use lenses to view their work instead of glass globes full of water, which distorted all but the center of the image: "spherical aberration". The lens, whose opposite surfaces were sections of spheres, took the place of the central parts of the globe over the image. He knew about magnetic poles attracting, if different and repelling, if the same, and the relation of magnets' poles to those of the heavens and earth. He calculated the circumference of the world and the latitude and longitude of terrestrial positions. He foresaw sailing around the world. He studied the planetary motions and astronomical tables to forecast future events. He did calculations on days in a month and days in a year which later contributed to the legal definition of a leap year. His explanation of a rainbow as a result of natural laws was contrary to theological opinion that a rainbow was placed in the heavens to assure mankind that there was not to be another universal deluge. Bacon began the science of chemistry when he took the empirical knowledge as to a few metals and their oxides and some of the principal alkalis, acids, and salts to the abstract level of metals as compound bodies the elements of which might be separated and recomposed and changed among the states of solid, liquid, and gas. When he studied man's physical nature, health, and disease, he opined that the usefulness of a talisman was not to bring about a physical change, but to bring the patient into a frame of mind more conducive to physical healing. He urged that there be experiments in chemistry to develop medicinal drugs. He studied different kinds of plants and the differences between arable land, forest land, pasture land, and garden land.