phic plates of various sensibilities. Monochromatic light is
utilized in photo-micrography for the best rendition of detail.
Light-waves have been utilized as standards of length because they are
invariable and fundamental. Numerous other interesting adaptations of
artificial light are in daily use.

This is in reality the age of artificial light, for mankind has not only
become independent of daylight in certain respects, but has improved
upon natural light. The controllability of artificial light makes it
superior to natural light in many ways. In fact, uses have been made of
artificial light which are impossible with natural light. Light-sources
may be made of a vast variety of shapes, and those may be transported
wherever desired. They may be equipped with reflectors and other optical
devices to direct or to diffuse the light as required.

Thus, artificial light to-day has numerous advantages over light which
has been furnished by the Creator. It is sometimes stated that it can
never compete with daylight in cheapness, inasmuch as the latter costs
nothing. But this is not true. Even in the residence, daylight costs
something, because windows are more expensive than plain walls. The
expense of washing windows is an appreciable percentage of the cost of
gas or electricity. And there is window-breakage to be considered.

In the more elaborate buildings of the congested portions of cities,
daylight is satisfactory a lesser number of hours than in the outlying
districts. In some stores, offices, and factories artificial light is
used throughout the day. Still, the daylighting-equipment is installed
and maintained. Furthermore, when it is considered that much expensive
area is given to light-courts and much valuable wall space to windows,
it is seen that the cost of daylight in congested cities is in reality
considerable. Of course, the daylighting-equipment has value in
ventilating, but ventilation may be taken care of in a very satisfactory
manner as a separate problem.

The cost of skylights in museums and other large buildings is far
greater than that of ordinary ceilings and walls, and the extra
allowance for heating is appreciable. The expense of maintenance of some
skylights is considerable. Thus it is seen that the cost and maintenance
of daylighting-equipment, the loss of valuable rental space and of wall
area, and the increased expense of heating are factors which challenge
the statement that daylight costs n