In the seventeenth century, the introduction of telescopes and microscopes and the advances in geometrical optics gave lens-making an ever-greater importance. The glass was made by melting siliceous sand with soda and other ingredients. A disk (called a blank) was cut from the block of glass, and its edge was shaped and smoothed. To produce convex lenses, concave lenses or other types of lenses, the disk was ground with a metal bowl whose surface was of the desired curvature. Abrasive damp emery powder was applied between the glass and the bowl. Other, much finer powders were used for the final polishing. In the seventeenth century, the masters of this process were Eustachio Divini and Giuseppe Campani, whose lenses were valued throughout Europe. Scientists such as Christiaan Huygens or Evangelista Torricelli often made their lenses themselves. Machines for semi-automatic lens-grinding were designed. Until the nineteenth century, however, the best lenses for optical instruments were hand-made; only those for non-specialized use, such as opera glasses, were machine-produced. Lens-making difficulties increased considerably with larger diameters. At the start of the nineteenth century, the limit for good lenses was about ten centimeters. Descartes and others had suggested lenses with non-spherical surfaces, but their production had been inhibited by tooling problems. Decisive progress in optical-glass production was achieved only thanks to the research of the German optician Fraunhofer, who, in collaboration with the Swiss glassmaker Guinand, developed a method to remove impurities and air bubbles in cast glass.