tion, or the measurable encroachment upon sound-shadows. In the accompanying diagram (Fig. 7) suppose the arrows to represent the direction of a group of parallel rays of either sound or light, the wave fronts being indicated by lines across the direction of the arrows. Waves in one phase are indicated by the continuous lines, and those in opposite phase by the dotted lines. At each edge of the obstacle are the centers of the secondary waves, whose fronts are represented by parts of circles. Behind the obstacle and on each side are points of interference represented by crosses
Fig. 7. — Exterior and Interior Diffraction.
and zeros. Behind it the secondary waves from opposite edges meet each other. At the sides, secondary waves interfere with the advancing main wave. Where like phases meet, the crosses represent points of increased disturbance. Where opposite phases meet, the zeros represent points of quiescence. If the waves are those of light, the crosses are points of increased brightness; the zeros, of comparative darkness. If the waves are those of sound, the crosses are points of noise; the zeros, of silence. Behind the obstacle there is a middle line of crosses; on each side of this a line of zeros; and outside of these are lines of crosses again. These lines are parts of hyperbolas, whose foci are the centers from which the secondary waves are started. This is readily seen by reference to the next illustration (Fig. 8). A necessary consequence is, that if light radiating from a point or a small aperture be interrupted by interposing a small disk in its path, there should be a line along the middle of the shadow behind it, at certain points of which brightness appears if a translucent screen is placed across the shadow. This fact was noticed by a Frenchman, Delisle, before the birth of either Newton or Huygens, but was of course not understood and was soon forgotten. Dr. Young seems not to have thought of it, or certainly never put this consequence