b=[.6961663 .4079426 .8974794]; 
l1=.6:.005:1.8;
m=length(l1);s=ones(1,m);s1=ones(1:m);
l=[.0684043 .1162414 9.896161]
nn=ones(1,3);
for j=1:m
for i=1:3

   nn(i)=b(i)/(1-(l(i)/l1(j))^2);
end
s(j)=1+sum(nn);
s1(j)=sqrt(s(j));
end
dif1=diff(s1);
dif2=diff(l1);
df=dif1./dif2;
xd=l1(2:m);
s2=s1(2:m);
mm=length(df);
for k=1:mm
ng(k)=s2(k)-df(k)*xd(k);
end
