the increase necessary in this case, we draw figure similar to the last,
or we use the formula given, and we find that we should multiply the
depth by 1.1, i.e. we should make the depth 6½ feet. If we prefer
to place this flush, we should have the same amount by increasing it 10
Example 6.--A room 40 feet wide, 50 feet long, 14
feet high, having light walls, and a reveal of 14 inches directly over the
window, to be used "For Storage," faces a 25 foot alley, opposite building
80 feet high.
Solution.--This is the same as problem No. 3, with
the exception of the reveal. In problem No. 3 we found a canopy necessary.
This introduces no change in the kind of quantity of prisms necessary. The
canopy should be set out to clear the reveal.
A Store Lighted From Both Ends.
In many stores it is desirable to place prisms
in each end. These cases may be considered in just the same way as two
rooms each lighted from one end, the total length of the two rooms thus
considered being equivalent to the long one. We may light the front
half of the room from the front end of the store, and the rear half from
the rear end, or we may light more than half from the front, say
two-thirds, and the remainder from the rear. The proportion of the
room which we will light from two ends depends very largely upon the
opportunities for light at those ends. The illumination would not be
changed to any extent if a white wall there thrown across the room at
this imaginary division line. Special attention is called to the
fact that simply one end of a store or room cannot be lighted
by the area of prisms suggested in the table, unless this end is
separated from the remainder by a white wall. If this is attempted
the areas of the tables should be increased by 20 per cent.
Example 7.--A room 25 feet wide, 90 feet long,
17 feet high, to be used "For Fine Merchandise," with no particular
obstructions, is to be lighted from both front and rear. The front
faces a street 65 feet wide, opposite building 50 feet high. The rear
faces a 25-foot alley, with opposite building 80 feet high. Prism
plate, 10 feet from sidewalk.