The platinum isotope 195 has an odd atomic weight and an even atomic number, that is the nucleus contains an odd neutron. It has been shown elsewhere by one of us that the distribution of nuclear spins in the nuclei with an odd neutron is totally different to that in the nuclei containing an odd proton. The number of mechanical moments known for those nuclei with odd neutrons is considerably less than that known for the other type, and for this reason we undertook the examination of the arc spectrum of platinum with a view to ascertaining the nuclear spin from the fine structures. Dempster has investigated the isotopic constitution of platinum with the mass spectrograph and reports the following isotopes, 192, 194, 195, 196, 198, the relative abundances being such that 192 is extremely weak, 194, 195, 196 all about similar, and 198 somewhat weaker than these. It is to be expected from this that some of the arc lines of platinum will exhibit a fine structure pattern due to the magnetic splitting of the 195 isotope and at the centre of gravity of this pattern will be a strong central component due to the grouping together of the even isotope components. If there is even isotope displacement, the centre component will be split up into three, the intensities of which will be such that two are strong and one weak. (The 192 isotope is so weak that it is being entirely neglected in the whole of this discussion.)