VII. Photographic image of stratified discharges. A letter to Prof. Stokes, Sec. R. S., by W. Spottiswoode, M. A., Treas. R. S

My Dear Stokes,—I am sure that you will be interested to hear that Capt. Abney yesterday succeeded in photographing some of the phenomena which I had observed last year, with a revolving mirror, and which are described in my paper “On Stratified Discharges, III.” (Proceedings of the Royal Society, vol. xxv. p. 73). The success of the operation was due mainly to his skill, but partly also to the great brilliancy and long duration of the discharges from my large induction-coil, described in the ‘Phil. Mag.’ for February last. The tube used on this occasion was a small hydrogen-tube of conical form, the effect of which, as seen in a revolving mirror, is represented in fig. 3 of the paper above quoted. The photographic image was obtained, not by the use of a mirror, but by moving the sensitive plate across the field of view during the continuance of the discharge. In this the first result, the position of the striæ, their proper motion, their grouping in pairs of different actinic power, and consequently of colour (a phenomenon well known in hydrogen tubes), are distinctly developed. Other features, of which I reserve the description, are also noticeable.