The use of the microscope for photography also embraces an extremely wide and useful field. Photographs of minute objects enlarged as much as 5,000 diameters (25,000,000 areas) can be produced in this manner, and a per manent record made which can also be used for reproduction by the usual printing processes for the illustration of books, etc. Recently the movements of insects and other small creatures have been reproduced by a biographic micro scope, and moving pictures of the unseen world are now presented to audiences as a means of amusement and education.
The microscope was invented between 1590 and 1609— the honors being divided between Hans and Zacharias Janssen, two Hollanders, and that greatest of opticians, Galileo. From its early form, consisting simply of a double or plano-convex object lens with an eye piece of a single convex lens to magnify the image, it developed by gradual stages until the latter part of the 18th century, without becom ing much more than a toy for the amusement of dilettante. As a matter of fact nearly every form of accessory which is in use at the pres ent time was devised and used in some form, but the desire for the ornamental and extraor dinary rather than the practical was every where manifest. With the awakening of general interest in scientific investigation, the micro scope began to be used as a tool to accomplish heretofore impossible results. This led to more practical forms of construction, and at last to their production in large quantities and at a cost which placed them within the reach of laboratories and individuals. In 1820 Fraun hofer invented a complete system of compu tation for telescope lenses, and Seidel and Steinheil soon followed with formulas for photographic lenses. In 1840 the Gauss theory was introduced and workers in optics applied this theory to computing lenses for the micro scope. In the early 70's Professor Abbe of the Zeiss optical works at Jena produced com putations for microscope lenses which placed them in the front rank of scientific appliances.
Among the early American pioneers in the optical improvement of the microscope, the names of Robert Tolles and the two Spencers stand prominent. By their extraordinary man ual skill and knowledge of optical principles, they succeeded in producing lenses which, in the case of one by Tolles, had a focal length of 1/75 inch, the highest power objective which has ever been constructed. It was found in practice, however, that, through inability to in crease the angle of aperture, these lenses gave no advantage over those of longer focus, and it is now rare to find an objective of less than 1/16 inch focus. With the introduction of the new optical glasses by the Jena works in 1886 92, new corrections were possible with micro scopic systems, and the phosphoric acid and boric acid glasses were especially useful to the microscope makers. But the greatest advance was made by the discovery of fluorite as a lens material, permitting the flat curvatures neces sary in the correction of spherical aberration. The resolving power of microscope lenses was greatly increased and the problems of their manufacture materially simplified by the in vention by Tolles of his ((duplex hone° objec tive, a construction which was voted imprac tical by the experts of the time, but which has since superseded all others in the construction of high-power lenses. By the application of Tolles' correction formulas to the Jena glass lenses a new series of very superior micro scopic systems was produced in 1911 by Winkel of Gottingen, notable particularly for bright ness and sharpness of the image. See MACRDS COPY. CLINICAL; ULTRAMICROSCOPE.
Bibliography.— Carpenter, W. B., The Microscope and Its Revelations) (Philadelphia 1901) ; Gage, S. H., The Microscope) (Ithaca, N. Y., 1911) ; Hovestadt, H., 'Jena Glais and Its Scientific and Industrial Applications) (London 1902) ; Spitta, E. J., (Microscopy: The Construction, Theory and Use of the Microscope) (New York 1907); Zeiss, K., 'Microscopes and Microscopical Accessories) (Jena, Germany, 1906).