of lighting and the
foregoing statement is not meant to depreciate those achievements.
However, the incandescent filament lamp has many inherent advantages.
The light-source is enclosed in an air-tight bulb which makes for a
safe, convenient lamp. The filament is capable of subdivision, with the
result that such lamps vary from the minutest spark of the smallest
miniature lamp to the enormous output of the largest gas-filled tungsten
lamp. The outputs of these are respectively a fraction of a lumen and
twenty-five thousand lumens; that is, the luminous intensity varies from
an equivalent of a small fraction of a standard candle to a single
light-source emitting light equivalent to two thousand standard candles.

Statistics are cold facts and are usually uninteresting in a volume of
this character, but they tell a story in a concise manner. The
development of the modern incandescent lamp has increased the intensity
of light available with a great decrease in cost, and this progressive
development is shown easily by tables. For example, since the advent of
the tungsten lamp the average candle-power and luminous efficiency of
all the lamps sold in this country has steadily increased, while the
average wattages of the lamps have remained virtually stationary.

AVERAGE CANDLE-POWER, WATTS, AND EFFICIENCY OF ALL THE LAMPS SOLD IN
THIS COUNTRY

Lumens
Year Candle-power Watts per watt
1907 18.0 53 3.33
1908 19.0 53 3.52
1909 21.0 52 3.96
1910 23.0 51 4.42
1911 25.0 51 4.82
1912 26.0 49 5.20
1913 29.4 47 6.13
1914 38.2 48 7.80
1915 42.2 47 8.74
1916 45.8 49 9.60
1917 48.7 51 10.56

It will be noted that the luminous intensity of incandescent filament
lamps has steadily increased since the carbon lamp was superseded, and
that in a period of ten years of organized research behind the tungsten
lamp the luminous efficiency (lumens per watt) has trebled. In other
words, everything else remaining unchanged, the cost of light in ten
years was reduced to one third. But the reduction in cost has