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proper directions. This sheet of prisms has been termed the
LUCIDUX, and it is evident that the prisms which are
inserted in this lucidux depend wholly upon the conditions of the
pavement prisms and of the room itself. It is found necessary even
to vary the prisms in this lucidux from the top to the bottom, but
it is not necessary to vary them from side to side, so that the lucidux
needed consists of a plate of prism lenses arranged in rows across the
plate. In order to find the prescription for the lucidux needed for
any particular place, first turn to the page of the tables corresponding
to the height of the prism pavement above the basement floor and
the column which is headed by the number corresponding to the horizontal
distance between the lucidux and the outer pavement prism. The lucidux
prescription is found in this column, and the distance to the lower edge of
each prism lens from the surface of the pavement is indicated in inches at
the left hand column. Notice that if there is a considerable head beam,
the top of this lucidux must be cut off, always leaving the lower edge of
each lens exactly at the correct distance from the surface of the pavement.
For example, to find the prescription for the lucidux for a basement of
which the surface of the pavement prisms is eight feet above the floor
of the basement, and the horizontal distance from the lucidux to the
outer pavement prism is five feet, and the lower edge of the head beam
is twelve inches below the surface of the pavement. In this case we
turn to page 250 corresponding to the height of the
sidewalk 8 feet,
and to the column headed 5 feet; here we find the lucidux prescription
is J, J, J, K, K, L, L, M, M, N, O, O; leaving the bottom of prism plate
48 inches from the surface of the sidewalk pavement, but since the lower
edge of the head beam is twelve inches below the surface of the pavement
we must take off the upper three rows of lenses, making the lucidux
K, K, L, L, M, M, N, O, O. This, of course, gives us names of the lenses
in each row of the lucidux from top to bottom. The length of the lucidux
depends upon the width of the basement.
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