only in the horizontal direction. In the same manner
the luminous intensities of light-sources until a short time ago were
expressed in candles as measured in a certain direction. Incandescent
lamps were rated in terms of mean horizontal candles, which would be
satisfactory if the luminous intensity were the same in all directions,
but it is not. Therefore, the candle-power in one direction does not
give a measure of the total light-output.

If a source of light has a luminous intensity of one candle in all
directions, the illumination at a distance of one foot in any direction
is said to be a foot-candle. This is the unit of illumination intensity.
A lumen is the quantity of light which falls on one square foot if the
intensity of illumination is one foot-candle. It is seen that the area
of a sphere with a radius of one foot is 4 pi or 12.57 square feet;
therefore, a light-source having a luminous intensity of one candle in
all directions emits 12.57 lumens. This is the satisfactory unit, for it
measures total quantity of light, and luminous efficiencies may be
expressed in terms of lumens per watt, lumens per cubic foot of gas per
hour, etc.

Of course, the efficiencies of light-sources are usually of interest to
the consumer if they are expressed in terms of cost. But from a
practical point of view there are many elements which combine to make
another important factor, namely, satisfactoriness. Therefore, the
efficiency of artificial lighting from the standpoint of the consumer
should be the ratio of satisfactoriness to cost. However, the scientist
is interested chiefly in the efficiency of the light-source which may be
expressed in lumens per watt, or the amount of light obtained from a
given rate of consumption or of emission of energy. This method of
rating light-sources penalizes those radiating considerable energy which
does not produce the sensation of light or which at best is of
wave-lengths that are inefficient in this respect. That radiant energy
which is wholly of a wave-length of maximum visibility, or, in other
words, excites the sensation of yellow-green, is the most efficient in
producing luminous sensation. Of course, no illuminants are available
which approach this theoretical ideal and it is not likely that this
would be a practical ideal. Under monochromatic yellow-green light the
magical drapery of color would disappear and the surroundings would be a
monochrome of shades of this hue. Having no c