182 Professor R. W. Wood [May 19, 



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 Professor Eubens 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 centre 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 vapours shine with a brilliant light when exposed to .these 

 invisible rays. One of the most striking is the vapour of metalhc 

 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 brilliant cone of green light which marks the 

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

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

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

 completely the rays in question. 



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

 violet region which resembles the band at wave-length 589;'> shown 

 by dense sodium vapour. So powerful is this absorption that I have 

 detected it in the vapour of mercury at room temperature. It oc- 

 curred to me that this light instead of being absorbed might possibly 

 be re-emitted by the vapour 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 centre of the bulb, which was not heated. The bulb was then 

 photographed 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. 



If the object to be photographed gives off visible rays in addition 

 to the invisible ones, it is necessary to remove these by a suitable 

 screen or ray filter. We will begin by considering some remarkable 

 effects which are obtained when sunlit landscapes are photographed 



