\(n=\frac{sin(\frac{A+\delta_{min}}{2})}{sin(\frac{A}{2}})\)

\(n_C=\frac{sin(\frac{60^0+38^0 20'}{2})}{sin(\frac{60^0}{2})}=1.51323\)

\(n_D=\frac{sin(\frac{60^0+38^0 55'}{2})}{sin(\frac{60^0}{2})}=1.51570\)

\(n_F=\frac{sin(\frac{60^0+39^0 12'}{2})}{sin(\frac{60^0}{2})}=1.52308\)

\(\Delta=\frac{n_F-n_C}{n_D}=0.01909\)