OJY RADIATION. 461 



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 ob- 

 tained. 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 envel- 

 ope, 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 produc- 

 tion 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 circum- 

 stances, 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 uj>on 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 re- 

 sembles 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 push- 

 ing in the screen, the whole ultra-violet spectrum falls upon it, and is 

 rendered visible from beginning to end. The spectrum is not con- 

 tinuous, but composed for the most part of luminous bands derived 

 from the w T hite-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. 



