His powerful scientific imagination enabled him to realize that all the points of a wavefront originate partial waves, the aggregate effect of which is to reconstitute the primary disturbance at the subsequent stages of its advance, thus accomplishing its propagation; so that each primary undulation is the envelope of an indefinite number of secondary undulations.

17 APQ is the arc of the circle representative of the wavefront of resolution, the centre being at 0, and the radius OA being equal to a.