To put these light values back together again at the distant receiving
station to make up a negative of the picture being broadcast from the
sending station, it is only necessary to reverse the process; first,
with a point of light to draw lines across a photographic plate, which
the rotating prismatic rings do; and, second, to vary the density of the
different parts of the successive lines corresponding to lights and
shadows of the picture at the sending station, and this the varying
strength of the incoming radio signal does by varying the intensity of
the light.
Dense areas in the negative are built up where the light is successively
very bright at the same place in adjacent lines; halftones where the
light is less intense; while where the light is very faint, little or no
exposure occurs, and shadows will result.
It is thus the lights and shadows which make the picture are built up,
line by line, for when this negative is developed, and paper prints made
therefrom, the dense areas produce high-lights in the picture; the less
dense areas the halftones; and the thin areas the shadows of the
picture, person or scene broadcast at the sending station. It is simply
that a photographic negative has been made of what the lens at the
sending station is looking at.
So, then, to receive pictures by radio, it is only necessary (1) to
cover a photographic plate in parallel adjacent lines, and (2) to vary
the density of the lines, to build up the shadows, the halftones, and
the high-lights of the picture.
If one puts a nickel under a piece of paper and draws _straight_ lines
across it with a dull pencil, a picture of the Indian appears. And that
is exactly the way photographs by radio are received, except that a
photographic plate is used instead of a piece of white paper, and a
pencil of light instead of the pencil of lead, the light pencil changing
the exposure in various parts of the successive adjacent parallel lines
by reason of the variation of the incoming radio signals.
The scheme is just a long camera with miles instead of inches between
lens and plate. For example, the lens in Washington and its photographic
plate in Boston; with this exception, that the one lens in Washington
can put a negative on one, ten or one hundred photographic plates in as
many different cities at the same time, and at distances limited only by
the power of the broadcasting station, radio instead of light carrying
the image from lens to plate.
The time for transmitting a picture depends upon the size of the picture
and strength of light, say, from three to six minutes, using a filament
lamp as a source.
The radio photograph receiving instruments are rather simple and
inexpensive and, like a loudspeaker, can be attached to any standard
amplifying audio-radio receiving set.