exposed to the weather, the iron work should be of the lightest
construction, and great care should be exercised in keeping the glass
clean on the outside.
It is usually impossible to so plan apartment
buildings and flat buildings that the light shaft or court will be of
the form suggested for office buildings and stores, i.e., a square or
rectangle, of which the shorter side is only a few tenths of the height.
A very large number of such courts have been treated with prisms, but
these are of such an irregular shape that it is impossible to give a
rule for the size of the canopies similar to the rules given for courts
in office buildings. A rough rule, which is easily applied, is the
following: The light delivered by the "A" canopy, if there are no unusual
obstructions in the way, is roughly proportional to the product of the
area of the canopy multiplied by the area of the top of the light shaft,
divided by the square of the height of the light shaft above the prism
plate, all the dimensions being in feet.
Nc=(court area)×(canopy area) / (court height)²
In finding the area of the top of the court, if the court is very long,
it should not be considered in any direction for a distance greater than
one-third the height of the light court above the canopy. If the "C" or
the "1/C" canopy is used the result is multiplied by 0.6
and 0.8 respectively.
TABLE OF LUXFER PRISM LUCIDUXES EXPLAINED.
Tables on pages 246-265 show the prescription
for the luciduxes to be used in connection with Luxfer Prism pavements.
The light which is projected from pavement prisms alone cannot be thrown
in a horizontal direction, because the surface of the pavement itself is
practically horizontal, and the light from one prism would strike the
prism immediately in front of it. This light, therefore, must be thrown
in a direction which makes an angle to the horizontal, and under ordinary
circumstances would strike the floor of the basement within a few feet of
the front. In order to throw this light back into the basement and to
give a satisfactory illumination, it is necessary to provide a vertical
sheet of prisms, which receives this light and distributes it into the
basement in the
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