FIGURE 3.

The raindrop analogy. Ripples on a pond can arise in two ways, either by separate raindrops (A) or from splashes (B). In the former case, the two ripples are coincidental, in the latter, the ripples are causally related. (C) Geometry for computing the number of ripples (solid circles) around the central ripple (shaded circle) as a function of distance from the center. The dotted lines define the annulus whose inner radius is r and outer radius is r +Δr. (D) Schematically shows how distances between central spark and neighbors are computed. Each plane is a confocal image. Solid circles mark the position of the central spark first occurring at time t and the neighboring sparks first occurring at t+T. Position of the central spark on the t+T image is marked by the shaded circle. The value r is the Euclidean distance between the central spark and its neighbors.