THE meeting of the Royal Society, held on May 14, was devoted to a discussion on ultra-penetrating rays. In his opening address* Prof. H. Geiger outlined three main lines of experimental investigation, referring first to the results of direct absorption measurements, and in particular to the very beautiful measurements of Millikan and Cameron, and of Regener, which together furnish a complete series of ionisation measurements from an altitude of 5 km. down to a depth of 240 metres below the surface of Lake Constance. Since, in traversing 240 m. of water, a particle will make many hundreds of collisions with oxygen nuclei, it is highly improbable that the ionisation at these great depths is produced by corpuscular radiation. On the other hand, the shape of the lower portion of the absorption curve is in good accord with the assumption that the ionisation is produced by photons. Appropriate analysis yields the value µ = 0.02 metre-1 for the true absorption coefficient of the hardest component; which, according to the generally accepted formulæ, corresponds to a quantum energy of about 3.7 × 109 electron volts.