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Bausch & Lomb Optical Company Because of the large area usually covered in the projection Projection opaque objects, as compared with that of lantern slides, it is necessary to use lenses of two or more times the focal length of those used for lantern slides, in order to obtain images of the same size from both slides and opaque objects (see tables, page 18). In order to obtain satisfactory illumination, we consider it necessary to main- tain the ratio between the aperture and the focal length of the lens at about 1 to 4 or 5. Hence there are limits of focal length, beyond which, for optical reasons, we cannot go. Therefore, it is not practical to attempt the projection of opaque objects at the extremely long distances at which lantern slide projection may be satisfactorily undertaken. Required Conditions To accomplish really satisfactory results it is necessary to have a totally darkened room and a good quality of screen, one with either a white opaque or an aluminum coating. Except in the case of the Home Balopticon, intended for use at short projection distances, the only illuminants we recommend for the projection of opaque objects are the arc lamp, using from 25 to 35 amperes of current, and the 1000-watt, gas-filled stereopticon Mazda lamp. Microscopical Projection This form of projection has become a very useful agency in the sciences, in class room and general demonstration work. An efficient projection microscope is of great value in supplementing table work with individual instruments. With it an instructor can demonstrate to all the members of a class simultaneously objects which are not available for individual examination. He finds it a great convenience, too, when he wishes to explain a slide to the class as a whole or to present a specimen quickly for comparison and discussion by members of the class, also in reviewing past work or assigning work in advance. Illustrated lectures on micro- scopical subjects are rendered possible, time is saved and the students interest is stimulated. Optically considered a projection microscope is a projection Its Early lantern with an objective of short focus for displaying very small ° ngin objects greatly enlarged. Historically it dates back nearly as far as the “laterna magica” itself. We find the latter used as a projection microscope as early as 1665. Fahrenheit, inventor of the thermometer bearing his name, devised a projection microscope before the year 1736 and exhibited it in Amsterdam. Among those who saw this instrument of Fahrenheit’s was Lieberkiihn, the anatomist, who was greatly interested in it as an aid in his work. He accordingly had one made and took it with him to England, where it aroused considerable interest. Thus it came about that Lieberkiihn was wrongfully regarded as the inventor, although he never claimed the honor. 9