The reproduction is, in photographic language, a negative, a somewhat unsatisfactory feature of the method. A simple modification, however, of the electrical connections will reverse that, so that the reproduction shall be a positive. There are two ways of cutting off a current from any particular circuit. One is to interpose a resistance, through which current cannot pass in an appreciable quantity, and the other is to provide a second path for the current so much easier than the first that practically all the current will pass that way, leaving the first circuit, to all intents and purposes, free. It is as if a farmer wished to stop people passing across a certain field. Two methods would be open to him: one to put up a high gate over which no one would dare to climb, and the other to provide a short cut so much more pleasant and convenient than the old path that no one having the choice of the two ways would think of going the old way.
What the farmer would call a short cut the electrician calls a short circuit, and a short circuit is often a more convenient way of cutting off a current than a switch which interposes resistance. At all events, in a case like this, a short circuit enables that to be accomplished which would be very difficult by any other means.
In the apparatus as already described the battery had to drive the current along a long wire, terminating at the distant receiving instrument, whence the current returned via the earth. The foil and pen, acting as a kind of electrical "tap," controlled this. When foil and pen touched, the tap was open and current flowed. When the line of non-conductive ink interposed itself, the tap was off and the flow ceased.
But connect the battery directly to the wire, and place the foil and pen in a short branch circuit, and the whole thing is reversed. Then the opening of the "tap" sent current to the other end; now the opening of the tap causes it to flow round the short branch and leave the main wire. Then the closing of the tap stopped the current reaching the farther end; now it causes it to do so. In fact, the entire action of the apparatus is completely reversed, and the bare parts of the foil become represented by blank paper, while the insulating lines produce the marks. In short, a positive results instead of a negative.
Such was the scheme of Caselli years ago. It is mentioned here at some length, since the principle of it is largely re-used in an improved form in the most successful of modern apparatus for a like purpose.
It undoubtedly was a very excellent scheme, simple and effective, which ought to have succeeded; but it did not do so, for the sufficient reason that at that time knowledge of electricity and skill in constructing delicate mechanism were not so highly developed as they are to-day. For success, as has already been said, one thing was essential, and that thing very difficult to obtain—a perfect synchronism between one stylus and the other. If the one were but the slightest degree "out of step" with the other, failure followed inevitably.
So the electrical transmission of sketches dropped for the time being. More recently a perfectly successful solution of the problem has come in another way altogether. This apparatus, at first called the telautograph, but now known as the telewriter, it will be more convenient to refer to later.
Of modern systems for the transmission of pictures the most successful, probably, are the Korn telautograph and the Thorn-Baker telectrograph.
Both of these are able to transmit very fair reproductions of photographs besides line drawings. The difficulty with photographs is, of course, that many parts of them are not of equal blackness or whiteness, but shade off gradually from one into the other. Take the case of a simple portrait. Part of the subject's face will be pure white, while the side in shadow will be comparatively dark. There is no hard and fast line between the two, but by a gradation through an infinite number of shades the one tones into the other. How can it be possible to convey that, more or less mechanically, over a wire? The solution is due to the fact that the eye will blend together a number of distinctly different shades, if properly arranged, into a gradual change. Really the change is step by step, but the effect is apparently quite continuous. This can be seen in the "half-tone" illustrations in this book. Close examination will show that such a picture is cut up into small squares. In the pure white part the squares are invisible, while in the perfectly black parts, if there be any, they are so merged into one another as to be inseparable. But everywhere else in the picture it will be seen that there are squares each with a dot in the middle. In the darker parts the dots are large; in the lighter ones they are small. We get the effect almost of colour, although the picture is done entirely in black ink. The eye does not see the individual dots when we are just looking at the picture; we have to examine it very closely to find them. Yet they are there all the time, and it is simply the peculiar action of the eye which sees beautiful half-tones, shading imperceptibly one into another, whereas in real fact there are only a vast number of equidistant dots, all equally black.
We see, therefore, that it is possible to split up a picture of any kind into a number of small squares and to treat each square as being of equal darkness throughout. Then, if we can communicate by wire that particular degree of darkness to a distant station, where the small parts can be put together in their proper order and given their correct shade, the picture as constructed at the receiving end will be something like that at the sending end. And we have only to make the size of each separate square small enough to obtain a copy which will resemble the original very closely indeed.