faculty for observing phenomena often overlooked by others. He first observed a molecular shadow in 1880 and made records of his observations in the Edison Laboratory note book. Since that time he has examined over 600 lamps, which were made at various periods during thirteen or fourteen years, by twelve different manufacturers. (_Trans. Amer. Inst. Electrical Eng._, Mar. 21, p. 161.) Every one, more or less, exhibited the molecular shadow. It is a principle, therefore, that if the carbon filament has both legs in the same plane, a sciagraph of one of them will be produced. As the shadow is on one side of the bulb only, the molecules fly off from only one electrode, viz., the cathode. By means of photography, the effect is increased because of certain well-known principles. The figure heretofore referred to is taken from a photograph, but, of course, does not represent the sciagraph as well as the original photograph, in view of the loss of effect by re-production by the half-tone process. For further theoretical considerations, see the Institute paper referred to, where the matter was discussed by Profs. Elihu Thomson, Anthony and others. Independently of Mr. Hammer’s discovery, Prof. J. A. Fleming, professor of electrical engineering in the University College, London, England, discovered and studied the matter, and presented it before the _Phys. Soc._ of London, appearing about 1885 (_from memory_). The name “molecular sciagraph” is given by the author because it is an accepted explanation that the deposit is due to either molecules or atoms of the electrode, given off by evaporation (page 46, lines 5 to 10), or electrical repulsion (§ 61_a_, lines 22 to 25), or, as some hold, by mere volatilization by the intense heat of incandescence, or one or more combined; but electrical repulsion certainly has something to do with the rectilinear propagation, for the molecules are charged according to § 4. [Illustration] ------------------------------------------------------------------------ CHAPTER X -------