manipulated easily so as to cover the entire prism plate. The
particular kind of shades employed should be light in weight, and should
be perfectly white, hung with a roller at the bottom of the prism plate.
Prisms Inside Window.
The ordinary methods of installing prism
plates are: 1st, In sashes, replacing glass; 2d, in foriluxes, separate
sashes, flush with the face of the building; 3d, in canopy prism plates
set at angles to the vertical.
It is sometimes desirable to install prism plates
on the inside of the room, just back of the glass of a window. This
should never be done unless the reveal over the window is very small, or,
if there is a considerable reveal, the zenith-tangent must be very large.
The greatest demand for this kind of installation has been where the prism
plates have been used for temporarily lighting the room. In such places
the prism plate has been attached to the window casing by hinges, allowing
it to open and close. This hinging is especially convenient for cleaning.
It is to be noted that the light coming from the sky passes through the
glass in front of the prism plate, making quite an angle with the same, and
losing about 20 per cent of its intensity. This necessitates an increase
in the prism plate of about 20 per cent in addition to any increase called
for by the presence of a reveal, as explained on page 88.
When Luxfer Prisms are placed in the fronts of
stores, either as transoms, as foriluxes or as canopies, it is very
essential that the arrangement of the store front, particularly the top
of the show window, should be such as to utilize the light to the best
advantage. It has been pointed out in several places, particular under
the head of foriluxes, that ordinary glass causes a loss of from 10 to
20 per cent of the light passing directly through it. This points at
once to the fact that the back of the show window should, if possible,
not be built up to the ceiling, because it not only deducts this 10
per cent of light due to the glass, but usually deducts a larger amount
due to the woodwork surrounding the same. It is