to compare the cost of lighting with the rental charge of the addition and of its equipment.  A fair rental value for plant and equipment is 50 cents per square foot per year; but of course this varies considerably, depending upon the type of plant and the character of the equipment.  An investigation showed that this value varies usually between 30 to 70 cents per square foot per year.  Using the mean value, 50 cents, it is seen that the rental charge is about seven times the cost of lighting.  Furthermore, there is a saving of 43 cents per square foot per year during the night operation by operating the night shift.  Of course, this is not strictly true because a depreciation of machinery during the night shift should be allowed for.  These fixed charges would average slightly more than half as much in the case of the two-shift factory as in the case of the same output from a factory twice as large but operating only a day shift.  Incidentally, the two-shift factory need not be a hardship for the workers, for, if the eight-hour shifts are properly arranged, the worker on the night shift may be in bed by midnight and the objection to a disturbance of ordinary hours of sleep is virtually eliminated.

In a discussion of light and safety presented in another chapter the startling industrial losses due to accidents are shown to be due partially to inadequate or improper lighting.  About one fourth of the total number of accidents may be charged to defective lighting.  The consumer bears the burden of the support of an unproducing army of idle men.  According to some experts an average of about 150,000 men are continuously idle in this country owing to inadequate and improper lighting.

This is an appreciable factor in the cost of living, but the greatest effectiveness of artificial lighting in curtailing costs is to be found in reducing the fixed charges borne by the product through the operation of two shifts and by directly increasing production owing to improved lighting.  The standard of artificial-lighting intensity possessed by the average person at the present time is an inheritance from the past.  In those days when artificial light was much more costly than at present the tendency naturally was to use just as little light as necessary.  That attitude could not have been severely criticized in those early days of artificial lighting, but it is inexcusable to-day.  Eyesight and greater safety from accidents are in themselves valuable enough to warrant adequate lighting, but besides these there is the appeal of increased production.

Outdoors on a clear summer day at noon the intensity of daylight illumination at the earth’s surface is about 10,000 foot-candles; in other words, it is equal to the illumination on a 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