1945 W.PAULI was discovered, I proposed to use the assumption of a nuclear spin to inter- pret the hyperfine-structure of spectral lines.T is proposal met on the one hand strong opposition from many sides but influenced on the other hand Goudsmit and Uhlenbeck in their claim of an electron spin. It was only some years later that my attempt to interpret the hyperfine-structure could be definitely confirmed experimentally by investigations in which also Zee. man himself participated and which showed the existence of a magneto- optic transformation of the hyperfine-structure as I had predicted it. Since that time the hyperfine-structure of spectral lines became a general method of determining the nuclear spin. In order to determine experimentally also the symmetry class of the nuclei, other methods were necessary. The most convenient, although not the only one, consists in the investigation of band spectra due to a molecule with two like atoms. It could easily be derived that in the ground state of the electron configuration of such a molecule the states with even and odd values of the rotational quantum nu umber are symmetric and antisymmetric respectively for a permutation of the space coordinates of the two nuclei. Further there exist among the(2 1+ 1)spin states of the pair of nuclei, (21+ 1)(+ 1) states symmetrical and (21+ 1)I states antisymmetrical in the spins, since the(2 1+ 1)states with two spins in the same direction are necessarily sym- metrical. Therefore the conclusion was reached: If the total wave function of space coordinates and spin indices of the nuclei is symmetrical, the ratio of the weight of states with an even rotational quantum number to the weight of states with an odd rotational quantum number is given by (1+ 1) 1. In the reverse case of an antisymmetrical total wave function of the iclei. the ratio is 1: (1+ 1). Transitions between one state with even and another state with an odd rotational quantum number will be extremely rare as they can only be caused by an interaction between the orbital motions and the spins of the nuclei. Therefore the ratio of the weights of the rotational states with different parity will give rise to two different systems of band spectra with different intensities, the lines of which are al- ternating The first application of this method was the result that the protons have the spin 12 and fulfill the exclusion principle just as the electrons. The initial difficulties to understand quantitatively the specific heat of hydrogen mole cules at low temperatures were removed by Dennison's hypothesis, that at this low temperature the thermal equilibrium between the two modifications gen molecule (ortho-H= odd rotational quantum numbers,34 1945 W.PAUL I was discovered, I proposed to use the assumption of a nuclear spin to inter￾pret the hyperfine-structure of spectral lines15. This proposal met on the one hand strong opposition from many sides but influenced on the other hand Goudsmit and Uhlenbeck in their claim of an electron spin. It was only some years later that my attempt to interpret the hyperfine-structure could be definitely confirmed experimentally by investigations in which also Zee￾man himself participated and which showed the existence of a magneto￾optic transformation of the hyperfine-structure as I had predicted it. Since that time the hyperfine-structure of spectral lines became a general method of determining the nuclear spin. In order to determine experimentally also the symmetry class of the nuclei, other methods were necessary. The most convenient, although not the only one, consists in the investigation of band spectra due to a molecule with two like atoms16. It could easily be derived that in the ground state of the electron configuration of such a molecule the states with even and odd values of the rotational quantum number are symmetric and antisymmetric respectively for a permutation of the space coordinates of the two nuclei. Further there exist among the (2 I + 1) 2 spin states of the pair of nuclei, (2 I + 1) (I + 1) states symmetrical and (2 I + 1)I states antisymmetrical in the spins, since the (2 I+ 1) states with two spins in the same direction are necessarily sym￾metrical. Therefore the conclusion was reached: If the total wave function of space coordinates and spin indices of the nuclei is symmetrical, the ratio of the weight of states with an even rotational quantum number to the weight of states with an odd rotational quantum number is given by (I+ 1) : I. In the reverse case of an antisymmetrical total wave function of the nuclei, the same ratio is I : (I + 1 ). Transitions between one state with an even and another state with an odd rotational quantum number will be extremely rare as they can only be caused by an interaction between the orbital motions and the spins of the nuclei. Therefore the ratio of the weights of the rotational states with different parity will give rise to two different systems of band spectra with different intensities, the lines of which are al￾ternating. The first application of this method was the result that the protons have the spin ½ and fulfill the exclusion principle just as the electrons. The initial difficulties to understand quantitatively the specific heat of hydrogen mole￾cules at low temperatures were removed by Dennison’s hypothesis17, that at this low temperature the thermal equilibrium between the two modifications of the hydrogen molecule (ortho-H2 : odd rotational quantum numbers