a brook is running in one direction as naturally as it can, and a greyhound is running in the opposite direction as naturally as he can, the combination of their contrary movements may not be pleasing in a motion picture. Art is art, not because it reflects some actual bit of nature, but because it is endowed with some beauty made by man.
What other properties pictorial motion should have, besides correct representation of action has been partly told in Chapter III, where the demands of ease and economy of vision were made a condition concomitant with beauty. We may further apply the same tests which have been applied to fixed design. But, in order to get a firm grasp of our subject let us first reduce pictorial motions to their simplest forms.
The simplest motion of all is the moving spot, especially when it is entirely unrelated to a setting or background; that is, the kind of moving spot which the spectator may see without at the same time seeing any other thing, either fixed or moving. A familiar example in nature is the dark dot of a bird flying high above us in a cloudless sky. An example from the screen is the effect of a ball of fire shot from a Roman candle through darkness, as in the battle scenes of Griffith's "Birth of a Nation." But even so simple a moving thing as a spot has two properties which are very important to the composer of motions. The moving spot, like all other motions, has direction and velocity. The buzzard soaring slowly in large circles affects us in one way, while the hawk swooping downward sharply, or the crow flying in a straight line, or the bat fluttering crazily in the air, affects us in quite a different way.
