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
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