Slowly the needle was tracing out a picture on the paper.  It was only a thin band yet, but gradually it was widening, though we could not guess what it was about to reveal as the ceaseless revolutions widened the photographic print.

“I may say,” explained Kennedy as we waited breathlessly, “that another system known as the Korn system of telegraphing pictures has also been in use in London, Paris, Berlin, and other cities at various times for some years.  Korn’s apparatus depends on the ability of the element selenium to vary the strength of an electric current passing through it in proportion to the brightness with which the selenium is illuminated.  A new field has been opened by these inventions which are now becoming more and more numerous, since the Korn system did the pioneering.

“The various steps in sending a photograph by the Baker telelectrograph are not so difficult to understand, after all.  First an ordinary photograph is taken and a negative made.  Then a print is made and a wet plate negative is printed on a sheet of sensitised tinfoil which has been treated with a single-line screen.  You know a halftone consists of a photograph through a screen composed of lines running perpendicular to each other—­a coarse screen for newspaper work, and a fine screen for better work, such as in magazines.  Well, in this case the screen is composed of lines running parallel in one direction only, not crossing at right angles.  A halftone is composed of minute points, some light, some dark.  This print is composed of long shaded lines, some parts light, others dark, giving the effect of a picture, you understand?”

“Yes, yes,” I exclaimed, thoroughly excited.  “Go on.”

“Well,” he resumed as the print widened visibly, “this tinfoil negative is wrapped around a cylinder at the other end of the line and a stylus with a very delicate, sensitive point begins passing over it, crossing the parallel lines at right angles, like the other lines of a regular halftone.  Whenever the point of the stylus passes over one of the lighter spots on the photographic print it sends on a longer electrical vibration, over the darker spots a shorter vibration.  The ever changing electrical current passes up through the stylus, vibrates with ever varying degrees of intensity over the thousand miles of telephone wire between Chicago and this instrument here at the other end of the line.

“In this receiving apparatus the current causes another stylus to pass over a sheet of sensitised chemical paper such as we have here.  The receiving stylus passes over the paper here synchronously with the transmitting stylus in Chicago.  The impression which each stroke of the receiving stylus makes on the paper is black or light, according to the length of the very quickly changing vibrations of the electric current.  White spots on the photographic print come out as black spots here on the sensitised paper over which this stylus is passing, and vice versa.  In that way you can see the positive print growing here before your very eyes as the picture is transmitted from the negative which Clark has prepared and is sending from Chicago.”