## Abstract

Accurate measurements of the paramagnetic resonance spectra of gadolinium and neodymium ethyl sulphates have been made both in strong magnetic fields at a wave-length of 3 cm, and in weak fields at wave-lenghts between 6 and 22 cm. (i) The Gd$^{3^{+}}$ ion is in an $^{8}$S state, whose levels are split by the action of the crystalline electric field, which is assumed to have C$_{3h}$ symmetry. The results are consistent with this supposition, except for some discrepancies in the position of the zero field lines, whose origin is not certain. The main parameters in the spin Hamiltonian are evaluated, and the spectroscopic splitting factor is found to be isotropic at 1$\cdot $990 $\pm $ 0$\cdot $002. (ii) The Nd$^{3^{+}}$ ion is in a $^{4}$I$_{\frac{9}{2}}$ state, which is split by the crystal field leaving a Kramers doublet as the ground state. The hyperfine structure due to the two odd isotopes 143, 145 has been measured and gives the ratio of the nuclear magnetic moments (143/145) as 1$\cdot $6083 $\pm $ 0$\cdot $0012. Some small discrepancies in the positions of the hyperfine lines in zero field are found, which prevent the determination of accurate values of the nuclear electric quadrupole interaction. The theory of Elliott & Stevens (1953b) leads to values of 1$\cdot $0 and 0$\cdot $62 nuclear magnetons for the moments of isotopes 143 and 145 respectively, with an uncertainty of about $\pm $ 25% arising from lack of a precise value for $\overline{r^{-3}}$, where r is the electron-nuclear distance.