22 SENSITIVE FLAMES AND SOUND-SHADOWS. 



tion, or the measurable encroachment upon sound-sliadows. 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 



Fio. 7. — Exterior and Interior DirrRACTioN. 



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 conse- 

 quence 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, De- 

 lisle, 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 



