EXPERIMENTS WITH INVISIBLE LIGHT—WOOD. 157 
Practically all sources of light in ordinary use give out more or 
less ultra-violet light which plays 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. Rubens 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 shining 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, which 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 brilhant 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 this absorption that I have 
detected it in the vapor of mercury at room temperature. It occurred 
to me that this 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 are (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. 
