SUNBEAMS AND THEIR WORK. 49 
cal rays will go on working after you have taken the 
lace away, and all the paper will become brown and 
your picture will disappear. 
The action of the photographic rays was well 
known long before I delivered these lectures, twenty 
years ago. 
But since some still more marvellous and wonder- 
working rays have been discovered. These rays were 
studied and their curious action first shown by Profes- 
sor Rontgen, of Wiirzburg ; therefore they are some- 
times called the Rontgen rays, and sometimes the 
X-rays, because X stands in algebra for an unknown 
quantity; and although we know how these rays act, 
we do not yet know what they are, except that they 
are not ordinary forms of heat, light, or electricity. 
They are produced by inserting platinum wires, 
one at each end, into a glass tube from which the air 
has been withdrawn so as to make almost a perfect 
vacuum. These wires are then connected with an 
electric battery, and a current of electricity at very 
high pressure is passed through the tube, producing 
a bluish-green light. But just before the current 
passes out at the other end of the tube, there is a dark 
space seen in which there is no bluish-green light. 
It is in this space that the X-rays lie. They are quite 
invisible in themselves, but if a screen is placed in 
their road, painted over with a fluorescent substance 
(such as the luminous paint put on matchbox cases), 
they set up vibrations in the paint which cause it to 
glow brilliantly. 
Now comes the wonderful part. If you make this 
screen of cardboard or wood, and turn the painted side 
