The screen used is sometimes the Bunsen disc. This is a disc of paper
with a spot of paraffine wax in the centre melted thoroughly into the
paper or with a ring of paraffine wax surrounding the untouched centre.
When this disc is equally illuminated on both sides the spot is nearly
invisible. Inequality of illumination brings it out more visibly.
Sometimes a Leeson disc is used. This consists of three pieces of paper,
two thin ones between which a thicker piece, out of which a star is cut,
is laid. When equally illuminated on both sides the star appears equally
bright on both sides.
The bar photometer is the standard form. A candle or pair of candles may
be burned at one end and an incandescent lamp at the other, or a gas
flame may first be rated by candles and used as a standard.
Synonyms--Bunsen's Photometer--Translucent Disc Photometer.
Fig. 259. BAR PHOTOMETER.
Photometer. Calorimetric.
A photometer in which the radiant energy, so called radiant heat, is
used as the measurer of the light.
In one type a differential air thermometer is used, one of whose bulbs
is blackened. On exposing this bulb to a source of light it will become
heated, and if lights of the same character are used the heating will be
in proportion to their illuminating power quite closely. The heating is
shown by the movements of the index. By careful calibration the
instrument may be made quite reliable.
Photometer, Dispersion.
A photometer in which the rays from one of the lights under comparison
are made more divergent by a concave lens. In this way a strong light,
such as all arc lamp can be photometered more readily than where only
the natural divergence of the beam exists. The law of the variation of
the intensity of light with the square of the distance is abrogated for
a law of more rapid variation by the use of a concave lens.
The diagram, Fig. 260, illustrates the principle. E represents a
powerful light, an arc light, to be tested. Its distance from the screen
is e. Its light goes through the concave lens L and is dispersed as
shown over an area A1, instead of the much smaller area A, which the
same rays would otherwise cover. Calling l the distance of the lens from
the screen, f its focus, and c the distance of the standard candle from
the screen when the shadows are of equal intensity, we have the
proportion.
Illuminating power of lamps: ditto of standard candle::
(l (e-l) + fe)2 : (c f)2