ACCORDING to Bohr's theory of atomic structure, the colour shown by compounds of elements belonging to the transition groups is ascribed to the presence of incomplete inner shells in them, and to the small difference in the binding energy of an electron attached to the outer valency and the inner incomplete shell respectively. Since the transference of an electron from the inner to the outer shell involves a change in its orbital moment, it is therefore to be expected that under the influence of light absorption the magnetic moment of the absorbing ion would also alter. One of us tried to observe this effect several years ago without any success, due, as it appears now, to the arrangement used not being sensitive enough.