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 p 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
