out with dominant brilliancy. To my surprise, at the very point where I expected this brilliant band to appear, a band of darkness took its place. By humoring the voltaic arc a little, the darkness vanished, and the bright band which I had sought at the beginning was obtained. On reflection the cause was manifest. The first ignition of the sodium was accompanied by the development of a large amount of sodium-vapor, which spread outward and surrounded, as a cool envelope, the core of intensely heated vapor inside. By the cool vapor the rays from the hot were intercepted, but on lengthening the arc the outer vapor in great part was dispersed, and the rays passed to the screen. This relation as to temperature was necessary to the production of the black band, for, were the outside vapor as hot as the inside, it would, by its own radiation, make good the light absorbed.
An extremely beautiful experiment of this kind was lately made here by Professor Liveing, with rays which, under ordinary circumstances, are entirely invisible. Professor Dewar and Professor Liveing have been long working with conspicuous success at the ultra-violet spectrum. Using prisms and lenses of a certain kind, and a powerful dynamo-machine to volatilize our metals, like Professor Liveing, I cast a spectrum upon the screen. Far beyond this terminal violet, waves impinge upon the screen which have no sensible effect upon the organ of vision; they constitute what we call the ultra-violet spectrum. Professor Stokes has taught us how to render this invisible spectrum visible, and it is by a skillful application of Stokes's discovery that Liveing and Dewar bring the hidden spectrum out with wondrous strength and beauty.
A small second screen is at hand, which can be moved into the ultra-violet region. Felt by the fingers, the surface of this screen resembles sand-paper, being covered with powdered uranium glass, a highly fluorescent body. Pushing the movable screen toward the visible spectrum, at a distance of three or four feet beyond the violet, where only darkness existed before, light begins to appear. On pushing in the screen, the whole ultra-violet spectrum falls upon it, and is rendered visible from beginning to end. The spectrum is not continuous, but composed for the most part of luminous bands derived from the white-hot crucible in which the metals are to be converted into vapor. I beg of you to direct your attention to one of these bands in particular. Here it is, of fair luminous intensity. My object now is to show you, with Professor Dewar's aid, the reversal, as it is called, of that band, which belongs to the vapor of magnesium, exactly as a moment ago you were shown the reversal of the sodium band. An assistant will throw a bit of magnesium into the crucible, and you are to observe what first takes place. The action is rapid, so that you will have to fix your eyes upon this particular strip of light. On throwing in the magnesium, the luminous band belonging to its vapor is cut away, and you have, for a second or so, a dark band in its place.