surface produced by a
light-source equivalent to 10,000 candles at a distance of one foot from
the surface. This will be recognized as an enormous intensity of
illumination. On a cloudy day the intensity of illumination at the
earth's surface may be as high as 3000 foot-candles and on a "gloomy"
day the illumination at the earth's surface may be 1000 foot-candles.
When it is considered that mankind works under artificial light with an
intensity of only a few foot-candles, the marvels of the visual
apparatus are apparent. But it should be noted that the eyes of the
human race evolved under natural light. They have been used to great
intensities when called upon for their greatest efforts. The human being
is wonderfully adaptive, but it could scarcely be hoped that the eyes
could readjust themselves in a few generations to the changed conditions
of low-intensity artificial lighting. There is no complaint against the
range of intensities to which the eye responds, for in range of
sensibility it is superior to any man-made device.

For extremely low brightnesses another set of physiological processes
come into play. Based purely upon the physiological laws of vision it
seems reasonable to conclude that mankind should not work under
artificial illumination as low as has been considered necessary owing to
the cost in the past. With this principle of vision as a foundation,
experiments have been made with greater intensities of illumination in
the industries and elsewhere and increased production has been the
result. In a test in a factory where an adequate record of production
was in effect it was found that an increase in the intensity of
illumination from 4 to 12 foot-candles increased the production in
various operations. The lowest increase in production was 8 per cent.,
the highest was 27 per cent., and the average was 15 per cent. The
original lighting in this case was better than that of the typical
industrial conditions, so that it seems reasonable to expect a greater
increase in production when a change is made from the average inadequate
lighting of a factory to a well-designed lighting-system giving a high
intensity of illumination.

In another test the production under a poor system of lighting by means
of bare lamps on drop-cords was compared with that of an excellent
system in which well-designed reflectors were used. The intensity of
illumination in the latter case was twenty-five times that of the former
an