156 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
waves discovered by Schumann, which are powerfully absorbed by 
air, might possibly render the air fluorescent, the emitted light being * 
invisible, however, on account of its short wave length. A heavy 
spark discharge was accordingly placed behind a small disk of metal, 
which cut off 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 neighborhood of the spark 
actually did give off actinic invisible rays, the photograph giving. 
the impression of a luminous fog surrounding the metal disk. 
I will now show you an experiment which illustrates that two objects 
which can not be distinguished under ordinary Ulumination may 
appear quite different when the light which illummates them is 
restricted to certain regions of the spectrum. I have here two pieces 
of scarlet silk which can not be distinguished the one from the other 
in the light of the incandescent electric lamps which uluminate this 
room. I now extinguish the lamps and place the two pieces of silk 
under this Cooper-Hewitt mercury are 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 
colored is fluorescent under the action of the green rays from the 
lamp; the red light is manufactured, so to speak, from the green 
light by the coloring matter of the silk. If I place the are 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 Jens, 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 suk upon which I have made some red spots with this same 
dye. By the ordinary iiumination 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 are 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 coloring 
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. 
