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.