Under average conditions, the direct light from the sky entering a room through sheet or plate glass strikes the floor within a few feet of the window and is mostly absorbed. For this reason, the rear part of the room is dark. By the installation of prisms at the window openings, however, the direction of the entering light is changed and projected horizontally into the room. Prisms do not create light; they simply distribute and diffuse the excess light at the window. Under certain conditions the effective light in a room may be increased fifty times by the installation of prisms. In Fig. 163, the arrows _a_ show the direction of the lowest rays from the sky over the cornice of the opposite building, striking the prism plate at the floors. These arrows also indicate vertical rays, striking canopies and skylights, while the arrows _b_ show the directions of the rays just mentioned after passing through the prism plate; also, the manner in which the light rays are projected into the building. [Illustration: FIG. 163] =168. Vault Lights for Basement, or Cellar, Lighting.=—Where the sidewalk and the basement ceiling are nearly on a level, basement rooms can be successfully lighted by the installation of =vault lights=, or _glass pavement tile_. The ordinary method is to use plain, round, or square lenses, which are set in iron or concrete in iron frames, as illustrated in Figs. 164 and 165, though vault lights are now made with the supporting material of reinforced concrete. Where the ceiling is low or the room at all deep, the system in which plain lenses are used is unsatisfactory, as the direct light from the sky in passing through the lens strikes the floor immediately underneath. [Illustration: FIG. 164] [Illustration: FIG. 165] [Illustration: FIG. 166] [Illustration: FIG. 167] The more modern method is to use prism tile set in the necessary frames. The prism receives the direct light from the sky on the upper face and turns, or refracts, it back into the basement, as illustrated in Fig. 166. Where the head-beam or other obstructions interfere with the deflection of light direct from the prism, it becomes necessary to use in connection with the skylight or canopy just described an independent vertical frame of window prisms of varying angles. These prisms intercept the light from the pavement tiles and project it into the basement, thus utilizing the light to the maximum advantage. =169. Glazier’s Points.=—A necessary adjunct in glazing with putty is the sheet-zinc, triangular =glazier’s points=, as illustrated in Fig. 167. These points are made in six sizes, which range from No. 000, the largest, to No. 3, the smallest. They are also made in somewhat similar shape, called _sharps_, but this type is not so popular as the _triangle point_.