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than 50°, the "1/C" canopy can be used, which stands at about 62° to the vertical. In this case, the canopy areas given in the columns to the right of the black line of the tables, when multiplied by 1.3, become the corresponding areas for the "1/C" canopy.
    It is to be understood that the areas given in the tables are designed to produce a certain result in a room. Four different results have been obtained by prescribing different amounts of product, and these have been classified according to the general use for which the room is designed. It is, therefore, easily seen that it is not necessary to follow these areas rigidly, but a slight deviation from them is allowable if the circumstances of the building render it desirable. The effect of such deviation will be a slight change in the illumination obtained. By reducing the quantity given in the table "For Fine Merchandise" to any great extent, the illumination obtained will be reduced to the class "For General Merchandise," or if it is increased to any great extent the illumination will belong or at least approach the illumination "For Desk Work."


Examples Illustrating Use of Tables.

    Example 1.--A room 20 feet wide, 60 feet long, 13 feet high, having light walls, to be used "For General Merchandise," with no particular obstructions, to be lighted from one end, facing a street 50 feet wide, opposite building 40 feet high. Required, the area and kind of prisms.
    Solution.--Turning to the table for zenith-tangents, we find that the zenith-tangent in this case is 1.25 at the street level, but since the bottom of the prism plate will be at least ten feet from the ground, we take 30 as the height of the opposite building, instead of 40 feet, and our zenith-tangent is 1.66.
    In order to give a good light "For General Merchandise," the arrangement of the store being not yet settled upon, we shall be safe if we throw the highest light 5° above the horizontal. We now turn to page 190, corresponding to "Highest Light 5° above Horizontal." We shall be safe for this "General
  Pocket Hand-Book of Electro-Glazed Luxfer Prisms - Page 86