8 
ee gren ( le +- my + “) 
rae Fas ’ 
Ko fo 70 
where /, m and n represent the direction-cosines of the normal 
to the wave-front, while ¢ is the time and s the normal velocity of 
propagation. fo, g, ete. are all constants. 
If the substitution is carried out, we find: 
f=—(nB —my), . its bop ae Se en 
g=—(ly —na), . ANNEER EN 1005 
h=—(ma—16), . enten BAE al 
—a=—(nQ—mR). sje tgs ae 
—b=—(1R—nP),. mee) wh Pern Te 
—c=—(mP—1Q), . dir eT, 
BEEN te ete =O Ca i wt a et oh te 
ke = mB nosey sa er Eet. (34) 
The last two equations are a direct consequence of the preceding 
formulae. Equations (27) to (32) may also be written as follows: 
Wass sin (ke By Ue) eee 
Te | ey 
Vv 
ee 5 en | 1 F 2 ° ° . ° ° 
ah, Ey (36) 
