IN a recent issue of NATURE (January 27, p. 114). Messrs. Ryan and Harkins have published some photographs of the ionisation tracks of recoiling atoms produced by collision of -particles with air molecules. We have been also engaged in photographing the tracks of -particles from polonium in helium, and have obtained some interesting photographs. Besides the long range recoil helium atoms, we have obtained a few photographs in which are shown the ionisation tracks of all the constituent parts of a helium atom, namely, of the nucleus and the two bound electrons. They are shown in Fig. 1 (i and ii). It will be noticed that both the electrons are ejected on the same side of the -particle track. One of us (D. Bose, Zeit. f. Phys., 12, 207, 1922) has previously photographed the ionisation tracks of several thousands of -particles in hydrogen, and in no case was a photograph obtained which showed simultaneously the ionisation tracks of the two constituents of a hydrogen atom. This behaviour can well be explained on the model of the normal helium atom proposed by Lande and others, according to which the two electrons move in orbits which are inclined to one another. If an -particle strikes the atom at the moment when both the electrons are near the point where their orbits cross, then the probability of their both being ejected in the same direction is very great.