brilliant manner the space within a few feet from the front, just
the same as it did before the prism plate was installed. This area
receiving good light extends about as far back from the front as 1½
times the distance from the floor to the bottom of the prism plate. The
remainder of the store must be lighted from the prism plate, and this is
usually accomplished by throwing the lowest light at an angle of either
40° or 45° below the horizontal, so as to overlap this front area
which is lighted directly from the sky. In many rooms it is not necessary
to light this front area: in these cases we do not throw the lowest light
so low, we draw upon our section a line from the center of the prism plate
to the nearest object which we wish to illuminate. We have found above
the direction of the highest light in the room, and we have now found the
direction of the lowest light. The prism plate, as described in the
TABLE OF PRESCRIPTIONS, throws light in all
directions intermediate between these two. In the illustration shown,
the highest light goes 5° below the horizontal, and the lowest light
35° below the horizontal.
In order to use the table, having drawn our
section and found the direction of the highest light, we turn to the page
of the table corresponding to the angle of the HIGHEST
LIGHT, we find the column corresponding to the
LOWEST LIGHT, and we find the row across
this table corresponding to the ZENITH-TANGENT. The
prisms which should be used are found at the intersection of this column
and the row. The first letter which is in heavy type is the name of the
prism which forms the body of the prism plate. These prisms have been
called Major Prisms. In some cases it is necessary to insert other
prisms to throw some of the light as low down as the lowest direction.
These are in indicated by a smaller type, and have been called Minor
Prisms. if a moderate amount of light is needed near the front of the
room, the major prisms should constitute 90 per cent of the plate, and
minor prisms 10 per cent. If a brilliant light is needed in the front
part of the room, this proportion should be changed to 80 and 20. In
our particular illustration, we turn to page 191,
select the fourth column, and, assuming the zenith-tangent of 1.2, we
find that we should use N
and L prisms.
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