t, and that only parts of
the same amperage are used together on the same lamp. No advantage is
obtained by over-running a Nernst lamp, this only shortening its life
without increasing the light. Under normal conditions the average life of
the burner is about 700 hours.

The efficiency of the Nernst lamp is fairly high, being only 1.45 to 1.75
watts per c.p. The light given is remarkably steady, and the lamps are
adaptable for all voltages from 100 to 300. In one of the large type of
lamps for use on a 235-volt {102} circuit the burner takes 0.5 ampere at
215 volts, and the resistance 0.5 ampere at 20 volts, while one of the
smaller lamps for use on the same circuit takes 0.25 ampere at 215 volts
and 0.25 ampere at 20 volts for the burner and resistance respectively. The
burner and heater are very fragile, and should never be handled except by
the porcelain plate to which they are attached. The lamps burn in air and
emit a brilliant white light of high actinic power, the intrinsic
brilliancy (c.p./square inch) varying from 1000 to 2500, as compared with
1000 to 1200 for ordinary metal filament lamps, and 300 to 500 for carbon
filament lamps.

The chief advantage of the Nernst lamp from a photographic point of view
lies in the fact that it produces abundantly the blue and violet rays which
have the greatest chemical effect upon a photographic plate or film. These
rays are known as chemical or actinic rays, and are only slightly produced
in some types of incandescent electric lamps. Carbon-filament lamps are
very poor in this respect.

Because a light is visually brilliant it must by no means be assumed that
it is the best to use for purposes of photography, and this is a point over
which many photographers stumble when using artificial light. Many sources
of light, while excellent for illumination, have very low actinic powers,
while others may have low illuminating but high {103} actinic powers. A
lamp giving a light yellowish in colour has usually low actinic power,
while all those lamps giving a soft white light are generally found to be
highly actinic.

In addition to the actinic value of the source of illumination, the
photographic film used must be very carefully chosen, as the chemical
inertia of the sensitised film plays an important part in the successful
reproduction of the picture, and also, to a certain extent, affects the
speed of transmission. The length of exposure, the amount of light admitted
to the