( 884 ) 
compared with 1°, for all values of u lying within the region we 
are concerned with. Then n’—n,* may be replaced by 2n, (n — 11), 
and separating the real from the imaginary part, we find: 
uM \ 
n=N—= Ee nd (6) 
nv (tu aD *) 
or 3.0% 
it = (Oa) 
2n,v,(40? at ed) 
which formulae easily show the symmetry of the curves representing 
n and nx as functions of u. 
As to n, it is clear indeed that n —=n, for u =O, and that for 
some positive value of u (i.e. on the violet side of »,) m is smaller 
than 2, by a certain amount, while for an equal negative value of 
mw (on the red side) » is larger than n, by the same amount. For 
u=—'/,r', nm reaches a maximum: 
= AN. 
lars & 4 NP, 
and for u == + '/, v\ a minimum: 
Q 
ery Ne 4n,v,v 
re. ie 0 \ 
The attenuation coefficient nx has its greatest value — So at 
= 0-40 
the point «=O; passes the value eS ,, Which is half that of the 
N,V _P 
maximum, at w= + '/,»', ie. exactly there where the maximum 
