he development of light-sources, of course, any harmful effects of
gases formed by burning or chemical action must be avoided. Some of the
fumes from arcs are harmful, but no commercial arc appears to be
dangerous when used as it is intended to be used. Gas-burners rob the
atmosphere of oxygen and vitiate it with gases, which, however, are
harmless if combustion is complete. That adequate ventilation is
necessary where oxygen is being consumed is evident from the data
presented by authorities on hygiene. A standard candle when burning
vitiates the air in a room almost as much as an adult person. An
ordinary kerosene lamp vitiates the atmosphere as much as a half-dozen
persons. An ordinary single mantle burner causes as much vitiation as
two or three persons.

In order to obtain a bird's-eye view of progress in light-production,
the following table of relative luminous efficiencies of several
light-sources is given in round numbers. These efficiencies are in terms
of the most efficient (yellow-green) light.

Efficiency
in per cent.
Sperm-candle 0.02
Open gas-flame .04
Incandescent gas-mantle .19
Carbon filament lamp .05
Vacuum Mazda lamp 1.3
Gas-filled Mazda lamp 2 to 3
Arc-lamps 2 to 7
White light radiated by "black-body" 16
Most efficient white light 40
Firefly 95
Most efficient light (yellow-green) 100

The luminous efficiency of a light-source is distinguished from that of
a lamp. The former is the ratio of the light produced to the amount of
energy radiated by the light-source. The latter is the ratio of the
light produced to the total amount of energy consumed by the device. In
other words, the luminous efficiency of a lamp is less than that of the
light-source because the consumption of energy in other parts of the
lamp besides the light-source are taken into account. These additional
losses are appreciable in the mechanisms of arc-lamps but are almost
negligible in vacuum incandescent filament lamps. They are unknown for
the firefly, so that its luminous efficiency onl