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Bausch & Lomb Optical Company Balo Projection Lenses The recent development of apparatus for projecting opaque areas of large size and the adaptation of the Mazda lamp to this work, as well as to lantern slide projection, have placed new and heavy demands upon the pro- jection lenses used. For projecting opaque objects a lens of very large diameter is required to collect in sufficient quantity the rays re- flected from the lighted surface of the object. In lantern slide projection a lens larger than that used with the arc is required to utilize the entire image of the light source as pro- jected upon the lens aperture by the condensers. Other things being equal, the illumination depends on the diameter of the lens, the efficiency of two different lenses comparing inversely as the square of their respective diameters. To meet this situation our Scientific Bureau has computed the new series of Balo projection lenses. Balo lenses are corrected for chromatic and spherical aberration and give critical definition to the extreme edges of the field. Since an increase in the diameter of the lens greatly increases the difficulty of correcting for these aberrations, it may be seen that the new series represents a real achievement in the production of high grade lenses for optical projection. With two exceptions these lenses are used on all Balopticons. The lenses of 1-/8 and 2 5 /i6 inches diameter are intended particularly for lantern slide projection. The former is made in focal lengths up to 10 inches and its use should be confined to outfits equipped with arc lamp. For longer focal lengths and especially for use with a comparatively large light source, such as the Mazda lamp or acetylene burner, the 2%6-inch diameter should be chosen. With such light sources this lens gives an approximate increase in illumination of 40 per cent, over the 1^4-inch diameter lens. The larger diameters are especially constructed for the projection of opaque objects. They have a ratio between diameter and focal length which varies between 1:3.5 and 1:5. The 4-inch diameter lenses have a special adjustment to accommodate them to varying projection distances, especially when working close to the screen. These lenses are very accurately mounted in heavy brass tubes with rack and pinion focusing adjustment and are finished in black with white engraving. 100