36_a_. HAMMER’S PHOTO-ELECTRIC DUST FIGURES. _From personal interview._—According to experiments of Elster and Geitel, hereinafter noted, § 98, Hammer’s dust figures shown in the accompanying half-tone cut may possibly be accounted for on the principle of the discharge of negatively electrified bodies by light. Mr. William J. Hammer, _Mem. Amer. Inst. Elect. Eng._, has a historical collection of incandescent lamps (_Elect. Eng._, N.Y., April 29, ’96, p. 446.) which were arranged on shelves in a glass case standing obliquely in the sunlight about an hour a day. After the lapse of many months, the very fine dust within the case lodged upon the inner surface of the glass in such a manner as to produce oval dust figures corresponding somewhat to the shapes of the lamps and some of them, appear after reproduction by the half-tone process in the accompanying cut. When the figures are inspected closely and the circumstances are known, no one can doubt that the sun and lamps acted as agents in their formation. As to the correct explanation, the matter has not been sufficiently discussed by scientists (presented here for the first time) to enable the author to render the opinions of others, but it is of interest in connection with Roentgen rays and the discharge of electrified bodies by light. As a matter of course, the surfaces of the lamps would reflect the light in such a way as to make bright spots (movable, however, with the sun) upon the glass of the containing case, and if the latter were in any sense charged by negative atmospheric electricity, this light would cause a variable amount of dust to be attracted according to the intensity of the rays striking the glass. These remarks are in the nature merely of a suggestion of a hypothesis. The heavy curved black line in the cut is a part of the frame of the glass case. The incandescent lamps do not show, simply because the case was empty when the photograph was taken. That the figures were not due to chemical action was shown by rubbing off some of the dust with the fingers. Finger marks were pictured on the figures. Off hand, Mr. Hammer and Prof. Anthony intimate air convection by differentiation of temperature, as a possible cause. [Illustration: FIG. 1.—HAMMER’S DUST-FIGURE ON GLASS. § 36., p. 21. ] [Illustration: FIG. 2.—HAMMER’S HISTORICAL COLLECTION OF INCANDESCENT LAMPS, CONTAINED IN CASE HAVING THE DUST FIGURES. § 36, p. 21. ]