This is under the reveal DC at the height BC above the top of the prism
plate. AE is a line drawn from A towards the top of the opposite building,
making therefore an angle Z with the prism plate. The area given in the
table should be multiplied by the quotient of EC divided by ED, i.e.
(EC/ED). If we indicate the reveal DC by r, the
depth AB of the prism plate by d, and the height BC of the reveal
above the prism plate by h, then it is evident that the correcting
factor (EC/ED) which multiplies the area given in the table
is (d+h)zt / (d+h)ztr.
Example 4.A room 20 feet wide, 60 feet long, 13
feet high, having light walls, to be used "For General Merchandise," to be
lighted from one end, faces a street 50 feet wide, opposite building 40
feet high. There is a reveal of 16 inches immediately over the front
windows.
Solution.This is the same as No. 1, with the
exception of the reveal. We found before that we should use a plate
of M and J prisms 3 feet
deep. If we wish to place our prisms under this reveal to make up for
the shadow cast by the reveal, we shall need to increase the depth of
the plate. Draw a section through the prism plate AC and the reveal DC;
draw a line AB towards the top of the opposite building and find that the
depth must be multiplied by 1.36. We shall, therefore, need a prism
plate 49 inches deep.
If we prefer, we can place the prism plate in a
separate sash and set it flush with the face of the building. In this
case add 10 per cent to the depth of the plate. The selection between
these two is a matter of taste.
Example 5.A room 25 feet wide, 90 feet long,
17 feet high, having light walls, to be used "For Fine Merchandise," to
be lighted from one end, faces a street 65 feet wide, opposite building
50 feet high. There is a 10 inch reveal 6 inches above the top of the
windows. No other obstructions.
Solution.Our problem is the same as the second,
with the exception that we have a reveal 10 inches wide, 6 inches above
the prism plate. We found for the second problem that we needed a plate
of prisms 6 feet deep. In order to determine
