source of information for early chemists in the 16th century, among them Georgius Agricola, who published his great work De re metallica in 1556. His work describes the highly developed and complex processes of mining metal ores, metal extraction and metallurgy of the time. His approach removed the mysticism associated with the subject, creating the practical base upon which others could build. Chemist Robert Boyle is considered to have refined the modern scientific method for alchemy and to have separated chemistry further from alchemy. Although his research clearly has its roots in the alchemical tradition, Boyle is largely regarded today as the first modern chemist and therefore one of the founders of modern chemistry, and one of the pioneers of modern experimental scientific method. Although Boyle was not the original discoverer, he is best known for Boyle's law, which he presented in 1662: the law describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a closed system. Boyle is also credited for his landmark publication The Sceptical Chymist in 1661, which is seen as a cornerstone book in the field of chemistry. In the work, Boyle presents his hypothesis that every phenomenon was the result of collisions of particles in motion. Boyle appealed to chemists to experiment and asserted that experiments denied the limiting of chemical elements to only the classic four: earth, fire, air, and water. He also pleaded that chemistry should cease to be subservient to medicine or to alchemy, and rise to the status of a science. Importantly, he advocated a rigorous approach to scientific experiment: he believed all theories must be tested experimentally before being regarded as true. The work contains some of the earliest modern ideas of atoms, molecules, and chemical reaction, and marks the beginning of modern chemistry. Physical Optics In 1604 Johannes Kepler published Astronomiae Pars Optica (The Optical Part of Astronomy). In it, he describes the inverse-square law governing the intensity of light, reflection by flat and curved mirrors, and principles of pinhole cameras, as well as the astronomical implications of optics such as parallax and the apparent sizes of heavenly bodies. Astronomiae Pars Optica is generally recognized as the foundation of modern optics. Willebrord Snellius found the mathematical law of refraction, now known as Snell's law, in 1621. It had been published earlier in 984 AD by Ibn Sahl.