"
Certainly I do, but this time we won't go out of the known. I have our old space flyer which we took beyond the heaviside layer six years ago ready for action and we're going to look for the invaders this afternoon."
"How will we see them if they are invisible?"
"They are invisible to ordinary light but not to ultra-violet light. While most of the ultra-violet is deflected and flows around the ship or else is absorbed, I have an idea that, if we bathe it in a sufficient concentration of ultra-violet, some would be reflected. We are going to look for the reflected portion."
"Ultra-violet light is invisible."
"It is to the eye, but it can be detected. You know that radium is activated and glows under ultra-violet?"
"Yes."
"Mounted on our flyer are six ultra-violet searchlights. By the side of each one is a wide angle telescopic concentrator which will focus any reflected ultra-violet onto a radium coated screen and thus make it visible to us. In effect the apparatus is a camera obscura with all lens made of rock crystal or fused quartz, both of which allow free passage to ultra-violet."
"What will we do if we find them?"