in Chapter IV, also have the power of directing our attention to the point where a crossing is made.
Then let us suppose that the Indians build a fire, from which the smoke rises in a tall, thin column. That would constitute another line of motion. But would it emphasize or weaken the center of interest? It would, as a matter of fact, still hold our attention on the camp because of the curious law that, no matter in what directions lines may move, it is the point which they have in common that attracts our attention. Thus if we assume a landscape where there is only a single stream, with a camp at the upper end, and with smoke rising from a fire, we would still have emphasis on the camp, in spite of the fact that the two lines of motion are directed away from it.
The same curious power over our attention may be exercised by moving spots. If we see, for example, two ships sailing away on diverging courses, we immediately suppose that the ships are sailing out of the same port, and, even though we cannot see any sign of that port, our minds will search for it. So also in those electric advertisements where lines of fire, sprayed from a central source, rise and curve over into the various letters of a word, the emphasis is rather on the point where the lines originate than on any single letter or on the word as a whole. Electric signs, by the way, are surprisingly often examples of what not to do with motion if one desires to catch the eye and to strike deep into the mind and emotions of the observer. The most common mistake, perhaps, is the sign consisting of a word in steady light surrounded by a flashing border in which a stream of fire flows continuously from dusk till dawn. Our
