between the shutter and the tube several sheets of aluminium, cardboard, or an oak board 3 cms. thick, does not hinder the phenomenon; any possibility of an action of radiated heat, as such, is consequently excluded. A thin film of water completely arrests the rays; light clouds passing over the sun considerably diminish their action.
The "N" rays emitted by the sun can be concentrated by a quartz lens; by means of the phosphorescent substance, the existence of several foci is ascertained. I have not yet determined their positions with sufficient precision to speak of them here. The "N" rays of sunlight undergo regular reflection by a polished plate of glass, and are diffused by ground glass.
The "N" rays issuing from the sun increase the glow of a small spark and a small flame in the same manner as those emitted by a Crookes' tube, by a flame, or by an incandescent body. These phenomena are easy to observe, especially if use is made of an interposed sheet of ground glass, as indicated by me in a preceding communication. The use of a
