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Bausch & Lomb Optic al Company The vertical equipment consists of a 4 V ?-inch plano-convex condensing lensr mounted in the sliding plate with the plane side up, to serve also as a stage for the subject, and a projection lens with reversing mirror carried above it on a vertical rod. Microscopical Projection For microscopical projection (Figure 4) provision is made for attaching a care- fully planed optical bed to the front of the base by means of a strong but con- venient set screw. The microscope with mounting is then attached to the bed in front of the large projection lens, which serves as a condenser. The movable mirror is raised to permit a free passage of light. If it is desired to change to the projection of opaque objects, the microscope can be quickly swung out of the path of light by means of a swinging arm. We list two microscopes for use on this Balopticon: our Medium Microscope, with simple substage condenser, and our Large Projection Microscope (Horizontal Type), with three substage condensers. The former is most efficient when used with medium power objectives, while the Large Microscope is equally efficient with all powers, low, medium and high. With either one there is supplied an ex- tension sleeve with supplementary condenser, which serves in connection with the projection lens for opaque objects as a substitute for the front lens of the triple condensing system. Spectroscopical Projection Figure 5—Accessories for Projecting Spectrum and Complementary Colors with Universal Balopticon. The apparatus for the projection of the spectrum and complementary colors, shown in Figure 5, consists of a supplementary bed, an adjustable slit, carbon bisulphide prism, holder to take deflecting wedges, and condensing and projecting lenses. An absorption spectrum is projected by placing the absorbing medium in the path of light before the slit, the emission spectra by introducing various chemi- cal salts into the arc. A pure spectrum will be obtained every time if the special revolving carbon holders are fitted to the arc lamp, permitting new carbons to be used with each salt introduced. 62