Measurements have been made on the hyperfine structure of the $^{3}P_{0}\ensuremath{-}^{3}S_{1}$ group of the $\ensuremath{\lambda}5485$ line of ${({\mathrm{Li}}^{7})}^{+}$ with a Fabry-Perot etalon crossed with a 21 ft. Paschenmount concave grating. The method of using the etalon with an astigmatic grating is discussed. The results of the measurements on the Fabry-Perot patterns of the $^{3}P_{0}\ensuremath{-}^{3}S_{1}$ group indicates a nuclear spin of $\frac{3}{2}$ for ${\mathrm{Li}}^{7}$ and a nuclear magnetic moment of 3.29 times the theoretical magnetic moment of the proton.