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In the illumination of the eyeground with sodium light the short and ultrashort waves of the spectrum are eliminated and the monochromatic light thus obtained allows observation of the retina in one visual plane. Originated by Kleefeld in 1935, this illumination was tried by several authors, who used a lamp in which sodium was volatilized. There was not, however, a reliable model until the Zeiss-Osram lamp put on the market a few weeks before the war, a lamp in which the filament is covered with a layer of alkaline oxide. When hot, this filament emits a large number of electrons, which ionize the atoms of gas (usually neon) filling the glass tube of the lamp. In the beginning the light is orange-red, but when the lamp gets hot it changes to yellow. As the temperature may reach 270 F. the lamp is surrounded by a double glass tube and a