EXPERIMENTS WITH INVISIBLE LIGHT WOOD. 157 



Practically all sources of light in ordinary use give out more or 

 less ultra- violet light which piays.no part in vision, but which can be 

 rendered apparent in various ways. I have on the table a new 

 arrangement by which these rays can be separated from the visible 

 ones. The apparatus is practically identical with the device quite 

 recently used by Prof. Kubens and myself for isolating the longest 

 heat waves that have been discovered up to the present time. It 

 can be used as well for the isolation of the ultra-violet, since its 

 action depends upon the high refractive index which quartz has for 

 these two types of radiation. The source is, in this case, an electric 

 spark contained in this box, and the ultra-violet rays are brought to 

 a focus upon a small circular aperture in a cardboard screen. The 

 focal length of the lens is so much greater for visible light that these 

 rays do not come to a focus at all, but are spread over a circular area 

 of a diameter nearly half that of the lens. 



A' penny has been fastened to the center of the lens with wax, and 

 this shields the aperture from the cone of visible rays coming from 

 the central portions of the lens. If I hold a sheet of white paper 

 above the aperture you observe that it remains dark — that is, no 

 visible rays pass through to the paper; if, however, I substitute for 

 the paper this mass of uranium nitrate crystals, the presence of the 

 ultra-violet rays is made manifest, the crystals sinning with a brilliant 

 green light. 



Certain vapors shine with a brilliant light when exposed to these 

 invisible rays. One of the most striking is the vapor of metallic 

 mercury, winch I can show you by boiling the metal in this flask of 

 fused quartz placed above the aperture. The metal is boiling now, 

 and you can all see the brilliant cone of green light which marks the 

 path of the ultra-violet rays through the metallic vapor. If I hold 

 a thin sheet of glass between the aperture and the flask, you will 

 observe that the vapor instantly becomes dark, for the glass stops 

 completely the rays in question. 



The vapor of mercury exhibits an absorption band in the ultra- 

 violet region which resembles the band at wave-length 5893 shown 

 by dense sodium vapor. So powerful is tins absorption that I have 

 detected it in the vapor of mercury at room temperature. It occurred 

 to me that tins light instead of being absorbed might possibly be 

 reemitted by the vapor laterally in all directions. To test this point 

 I sealed up a drop of mercury in an exhausted flask of quartz, and 

 focused the light of the mercury arc (burning in a silica tube) at the 

 center of the bulb, which was not heated. The bulb was then photo- 

 graphed with a quartz lens, and the picture clearly showed the cone 

 of focused rays precisely as if the bulb were filled with smoke. This 

 is another very good example of how new discoveries may be made 

 by ultra-violet photography. 



