These have ordinarily an upper and
lower sash. Both, if small, must be fitted with Prisms to secure
adequate light. Otherwise, the Prisms are installed in a single
panel, made up to full glass size, in the upper sash—fitted in a
rabbet and fixed with ordinary stops like a sheet of plate.
The result is good when the window is
set well forward to the outer face of the building and when no mouldings
or overhang shades the top of the sash.
Luxfer Prisms do their
best work where the light from the sky or clouds falls fullest upon
them.
