92 
MACFARLANE. 
as the three axes are mutually rectangular, the above operator 
becomes for a scalar function 
_ 2== ^_ 
Sr 2 ?' 2 SO 2 Sz 2 r Sr 
It will be observed that a term in does not appear at 
3 
all, but that the term in disappears only when the func¬ 
tion does not involve p. 
