On the Existence, in the Rays emitted by an Auer Burner, of Radiations which traverse metals, wood, etc. (May 11, 1903).

A focus tube emits, as I have already proved (see p. 7), certain radiations susceptible of traversing metals, black paper, wood, etc. Amongst these, there are some for which the index of refraction of quartz is nearly 2. On the other hand, the index of quartz for the rays remaining from rocksalt, discovered by Professor Rubens, is 2'18. This similarity of indices led to think that the radiations observed in the emission of a focus tube would very likely be near neighbours of the rays discovered by Rubens, and that, consequently, they would be met with in the radiation emitted by an Auer burner, which is the source of such rays. I accordingly made the following experiment: an Auer burner is enclosed in a kind of lantern of sheet-iron, completely enclosed on all sides, with the exception of openings for the passage of air and combustion

gases, which are so arranged that no light escapes; a rectangular orifice, 4 cms. wide and 6.5 cms. high, cut in the iron at the same height as the incandescent mantle, is closed by a sheet of aluminium 1 mm. thick. The chimney of the Auer burner is of sheet-iron, and a slit 2 mms. wide and 3'5 cms. high is cut in it, opposite the mantle, so that the emerging luminous pencil is directed on the aluminium sheet. Outside the lantern, and in front of this sheet, a double-convex quartz lens is placed, having 12 cms. focal length for yellow light, behind which is a spark-gap of the kind already described, giving very small sparks. The spark is produced by a small induction-coil, provided with a rotating make and break device, which works with perfect regularity.

The distance of the lens from the slit being 26.5 cms., one notes, by help of the spark, the existence of a focus of very great sharpness at a distance, p', of about 13'9 cms. For at this point the spark exhibits a notably greater glow than at the neighbouring points, whether in front or behind, above or below, to the right or to the left. The distance of this focus from the lens can be

determined within 3 or 4 mms. The interposition of a sheet of lead or glass 4 mms. thick causes this action to disappear. By varying the value of p, other values of p' are obtained, and substituting these values in the lens formula, the number 2'93 is obtained for the refractive index, being the mean value derived from a series of determinations as concordant as the precision of such observations could entitle one to expect. Similar experiments, made with another quartz lens, having a focal length of 33 cms. for yellow light, gave for the index the value 2'942.

While pursuing these experiments, I ascertained the existence of three other species of radiations, for which the index of quartz has values 2'62, 2'436, 2:29 respectively. These indices are all greater than 2, which explains the following fact: if in the path of the rays emerging from the lens a quartz prism of 30° refractive angle is placed, in such a way as to receive these rays in a direction sensibly normal to one of the refracting faces, no refracted pencil is obtained.

The radiations from an Auer burner,

transmitted through an aluminium sheet, are reflected by a polished plate of glass in conformity with the laws of regular reflection, and are diffused by a plate of ground glass.

These radiations traverse all the substances whose transparency I tested, with the exception of rock-salt 3 mms. thick (note 4), platinum 4 mms. thick, and water. A slip of cigarette paper, which is completely transparent when dry, becomes absolutely opaque when wetted with water. Figs. 2 and 3 are reproductions of the impression made in four seconds on a sensitive plate, without any photographic apparatus, before and after wetting the sheet of paper interposed between the lens and the spark. The photoengraving, produced from a paper print, shows that in the first case the spark is notably brighter.

These photographic prints are produced by the spark influenced by the rays, and not by the rays themselves, these latter producing no appreciable photographic effect after an hour's exposure.

Amongst the bodies which are traversed, I