THE EEFRACTION OF SOUND. 



209 



It follows tliat to obtaiu a focal convergence by means of a bydrogen 

 lens, we sbould have to employ a concave form, as shown in Fig. 3. In 



1 2 



Fig. 2. — Hydrogen lens, (a.) 



this case, the outer annulus of the wave on entering the projecting sur- 

 face of the lens g^ as at 4 and 5, would be hurried forward into a con- 

 cave form, as shown at 5 and 6, having a general center of convergence 

 at about e. On emerging from the second surface, /t, (supposed here to 



Fig. 3. — Hydrogen lens, (&.) 



be a plane,) the wave, by oblique retardation, as shown at 7, would be 

 somewhat flattened, as at S, 0, 10, &c., extending the focal point of con- 

 vergence to/. The eflect of a double concave surface to the lens would 

 be to shorten the focal distance according to the degree of concavity. 



2. — REFRACTION FROM INEQUALITY OF WIND, 



In 1857, Prof. G. G. Stokes showed that differences of motion iu the 

 air must exert a bending influence on the beams of sound, and that this 

 S. Mis. 115 14 



