891 
‘Appendix 
25 
NOTE: An equation number which is underlined refers to an 
equation of the appendix. 
1. Paragraphs ].-§. of this avpendix are devoted to a discussion 
of the fundamental expression for the pressure (9.3). A common 
starting point of the diffraction theory is Kirchhoff's theorem: 
Let  (F,t) be a solution of the wave equation, 
Olea aaa 
whose partial derivatives of the first and second order are 
continuous within and on a closed surface 8. Let rt, be a point 
inside of 8. Then 
(uaa) y Red Pfc aca) -&, HB BE] albR]S 48 
where r ia the distance from r to dS, 2, denotes differ- 
entiation along the inward normal to S, and square brackets in- 
dicate retarded values, e.g., [y (t)] = (t-2). If, however, 
ry lies outside 8, the integral vanishes. The integrand in (1,1), 
which depends upon the position, #,, of the field voint and the 
position, z., of d3 will be denoted by 
(#cr,}. 
_2. Kirchhoff's theorem may be applied to a progressive wave 
moving into an undisturbed medium. Let 8, be that vart of the 
wave front which hae crossed a certain plane, T, and let 85 be 
that vart of T which the wave front has left behind. Let S= 3) 
= 8,- Since the retarded quantities CY! , pay) » and #) 
