Roger Bacon, an Oxford master, applied mathematical knowledge to natural phenomena such as metal work, mineral work, the making of weapons, agriculture, and the remedies and charms of wizards and magicians. 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. (However it was another 400 years before the discovery of the image on the retina.) He comprehended the magnifying power of convex lenses and conceptualized the combination of lenses which would increase the power of vision by magnification. Soon afterwards, eyeglasses were available to correct farsightedness. Bacon studied gravity and the propagation of force, specifically illustrated by the radiation of light and heat. 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. This was the beginning of the science of physics. 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 the general concept of generation of liquids, gases, and solids, which was the beginning of the science of chemistry. He made experiments that led the way to saltpeter being made to explode, which led the way to the formulation of gunpowder. He believed that the principle of explosive energy would one day carry ships across the seas without sails and propel carriages down the streets, and flying machines. He knew the power of parabolic concave mirrors to cause parallel rays to converge after reflection to a focus and was familiar with work done to produce a mirror that would induce combustion at a fixed distance. He studied man's physical nature, health, and disease, the beginning of the science of biology and medicine. He opined that the use of a talisman was not to bring about a change, but to bring the patient into a frame of mind more conducive to physical healing. Bacon studied different kinds of plants and the differences between arable land, forest land, pasture land, and garden land.