1*111] on Recent Experiments with Invisible Light. 181 



spark has been discovered. It was suspected that the very short 

 waves discovered by Schumann, which are powerfully absorbed by 

 air, might possibly render the air fluorescent, the emitted light lieing 

 invisible, however, on account of its short-wave-length. A heavy 

 spark discharge was accordingly placed behind a small disc of metal, 

 which cut ofiF all the direct light, and the surrounding region photo- 

 graphed with a quartz lens, which is transparent to the ultra-violet 

 rays. It was found that the air in the neighbourhood of the spark 

 actually did give off actinic invisible rays, the photograph giving 

 the impression of a luminous fog surrounding the metal disc. 



I will now show you an experiment which illustrates that two objects 

 which cannot be distinguished under ordinary illumination may 

 appear quite different when the light which illuminates them is 

 restricted to certain regions of the spectrum. I have here two pieces 

 of scarlet silk which cannot be distinguished the one from the other 

 in the light of the incandescent electric lamps which illuminate this 

 room. I now extinguish the lamps and place the two pieces of silk 

 under this Cooper-Hewitt mercury arc lamp, and as you see one of 

 them still appears scarlet as before, while the other appears very dark 

 blue, almost black, in fact. The peculiarity of the mercury lamp lies 

 in the fact that it gives out little or no" red light, consequently red. 

 objects in general appear almost black. The peculiarity of this 

 particular piece of silk, by virtue of which it appears quite as red as 

 in ordinary lights, lies in the fact that the red dye with which it is 

 coloured is fluorescent under the action of the green rays from the 

 lamp : the red light is manufactured, so to speak, from the green 

 hght by the colouring matter of the silk. If I place the arc lamp 

 and the piece of silk behind this large sheet of red glass, you will 

 observe that the fabric is actually brighter than the lamp itself, 

 probably eight or ten times as bright. We can form an image of the 

 lamp on the silk with a lens, and the image will be many times 

 brighter than the lamp, which might be taken as a refutation of the 

 old and well-known theorem in optics that no optical system can 

 yield an image brighter than the source (!) Here is another piece 

 of white silk upon which I have made some red spots with this same 

 dye. By the ordinary illumination of the room it is seen to be white. 

 with large pink polka dots, something quite suitable for a young lady's 

 summer gown ! I now place it behind the red screen under the 

 mercury arc and it at once becomes quite diabolical in appearance, 

 bluish-black with flaming spots of scarlet, entirely unsuitable for the 

 aforementioned purpose. The dye which was used for colouring 

 these fluorescent fabrics was rhodamin. The conditions of illumina- 

 tion and observation are, of course, rather special in these cases, and 

 I have introduced them merely to illustrate how the eye may be 

 deceived under certain conditions. 



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 



