140. DISCHARGE TUBE SURROUNDED BY A VIOLET HALO. By means of enormous potential and high frequency, the tube was surrounded, Tesla stated, by violet luminosity or halo. § 6. and 74. From the fact that Lenard obtained a similar appearance in front of the aluminum window, it might be reasonable to conclude that there is some close relationship between the two phenomena. As an illustration of halo by light, may be mentioned the well known appearance so often occurring in the atmosphere concentrically with the moon, and sometimes surrounding the sun. Under favorable circumstances, (in a mist or dust in the air), a halo, at some distance from a flame or other light is faintly visible. It has generally been assumed that the reason of a halo by light is based upon the laws of reflection, or refraction or both, the bending of the rays taking place, through, or upon the surface of the particles of moisture. Others have held that particles of ice in the upper atmosphere, are the reflectors or refractors, or both. More puzzling has been the attempt to explain the novel appearance reproduced fairly well in the cut, page 140. It is here represented in print for the first time, but the photograph from which it was taken, was at various times, shown to different physicists, some of whom attributed the beautiful effect to the property of interference of light, and naming Newton’s rings as an analogous production. Prof. Anthony in an interview expressed himself as well satisfied that interference could not occur under the circumstances named. He recognized that there was a curved surface of glass which might be considered as made up of an infinite number of layers. The author introduces the matter for the purpose of consideration by others, and especially because it is so intimately connected with the subject of the vacuum tube and electricity. The details must be understood for the purpose of proper appreciation. Mr. William J. Hammer, of New York, had a photograph taken of the large Concert Hall at the Crystal Palace, Sydenham, Eng., by the light of the Edison incandescent lamps with which the Hall was illuminated. This photograph was made in 1882 during the International Electrical Exhibition held at the Crystal Palace. The picture shows a small section of the whole photograph and represents (although probably no one would judge so by looking at the picture) a festoon of _pear_-shaped incandescent electric lamps, each one hanging downward, and separated from its neighbor by between _three and four_ feet. They were so far away from the camera that a picture of the lamps unlighted, would have represented them as mere specks. The bright circles with the bright central crosses in the centre of the dark spaces were, therefore, fully one foot in diameter, while the lamp bulbs themselves were only about two or three inches thick, as usual. Why then should there be the halos? Why should the crosses appear? And why should