With the invention of the telescope and the microscope early in the 17th century, the production of optical lenses became an important factor in the development of these instruments for scientific observation and investigation. The obstacles to the production of suitable lenses were not surmounted until the 18th century because of lack of knowledge of the optical properties of lenses, and the difficulties in producing glass of suitable clarity due to primitive grinding and polishing techniques. The early astronomical lenses were produced by means of the primitive equipment of the mirror-makers and polishers of pietre dure in Murano and Venice. Apparatus for lens grinding and polishing was developed by Ippolito Francini of Florence, improved by Eustachio Divini and Carlo Antonio Manzini. A major advance was made by Giuseppe Campani of Rome. Contributions were made by Christiaan Huygens in Holland and John Marshall in England. Craftsmen in England and France made great strides in the improvement of apparatus and techniques for lens grinding and polishing. However, optical workshop practice improved extremely slowly, and it remained virtually unchanged into the 19th century.