Applied optics
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An attempt is made to develop a set of empirical equations for the relationship between skylight illuminance, turbidity, and solar altitude under the condition of a cloud free sky. The maximum possible illuminance from a cloud-free sky is estimated using these equations. The illuminances are stated in relation to the illuminance produced by a Rayleigh sky. An improved approximation for Bemporad's airmass function is used in the computations.
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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. In spite of improvements in equipment and techniques, 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. ⋯ Other important contributions were made by Christiaan Huygens in Holland and John Marshall in England. Toward the end of the 17th century, craftsmen in England and France made great strides in the improvement of apparatus and techniques for lens grinding and polishing. In spite of them, however, optical workshop practice improved extremely slowly, and it remained virtually unchanged into the 19th century.