Doctor Wood. Modern Wizard of the Laboratory: The Story of an American Small Boy Who Became the Most Daring and Original Experimental Physicist of Our Day-but Never Grew Up

One of its departments made tests of suspected passports and other dubious documents for erased writing, superimposed writing, invisible inks, etc. They also tested similarly shirt fronts, cuffs, handkerchiefs, linen of suspected spies — even panties and petticoats if the suspected spies were female. These articles of apparel might have been written on with invisible ink — or they might have been treated with chemicals which could be used in making invisible ink when soaked in water — to be used elsewhere for invisible writing and later developed by another chemical. The British experts showed me all the various chemical methods in use for developing secret writing. They showed other interesting activities, and I was waiting with some anticipation to see what was going on inside a small cabinet with no window which stood in the middle of the laboratory with wires running through the wall and along the ceiling. I suspected its use, and finally, when no mention of the cabinet was made at all, I asked, “What goes on in there?”

“Oh, I’m sorry”, said the captain who was showing me about, “but that’s very secret. We don’t show that to anybody”.

“Ultraviolet light, I presume”, said I in a detached manner.

“What!” said the amazed captain. “What makes you think so?”

“Because I invented the method and the black glass that cuts off the visible light, and sent the formula to your Admiralty from our Science and Research Division over a year ago”.

“Will you wait a moment”, said the captain, “while I speak to the colonel?”

I was presently ushered, with suitable apologies, into the dark room.

They had a quartz mercury arc in a box, with a window of dark-blue cobalt glass, under which they placed a German passport. When you looked at it through a yellow glass plate which cuts off the blue light reflected from the paper, you could see here and there German words, not supposed to be on any passport, which gave off a small amount of yellow light when stimulated by the violet rays. I remarked that this was the method of detecting fluorescence employed by Sir George Stokes more than half a century before. I asked why they did not use ultraviolet light to start with, which produces a strong fluorescence and is invisible.

“I’ll show you what I mean”, I said. “Come back into the dark room”. I happened to have a small plate of my black ultraviolet glass in my pocket, and we fitted it before a hole in a sheet of cardboard and stood it in front of the lamp window. The passport was now seen to be covered with previously invisible writing, practically all of the German words shining with a pale blue light.

“But where can we procure those plates?” they asked.

“I don’t understand why you haven’t got them”, I replied. “Your government has them. I sent the formula over a year ago to the Admiralty. A lot of them have been made and are in actual use at your Portsmouth Navy Yard…

“Oh, but you know”, said they, “the liaison between our Navy and Intelligence Department is not as good as it might be. We’ll call up Portsmouth and see if they can supply us…”. Portsmouth obliged at once.

By this time, it seems, they were not only keenly absorbed but also a little on the defensive for the moment. So they proudly explained that they had devised a note paper on which it was impossible to inscribe secret writing. This paper had been on sale at all post offices, and letters written on it were not subject to the long delay necessary for applications of their various tests for secret writing. This paper had proved very popular, as the letters passed the censor immediately. It was ordinary note paper on which fine parallel lines had been printed close together, in pale red, green, and blue ink — the red being soluble in water, the green in alcohol, and the blue in benzine. (The paper looked gray to the naked eye.) Since practically all liquids employed for making invisible writing fall into one of these three classes, one set of colored lines must dissolve in the colorless fluid flowing from the pen and produce colored writing. I recalled I had discovered years before that the pigment Chinese white comes out black as charcoal in photographs made with ultraviolet light, so I said, “Suppose I write on it with a fine crayon of Chinese white; then none of these lines will dissolve, yet it can be read by photography”.

“Oh, no”, they told me, “you can’t even write on it with a toothpick or glass rod without making legible writing. The colored inks are made slightly soft or ‘tacky,’ so that they smear together and produce dark gray letters. Here, try to write on it with this glass rod”.

I tried to write invisibly with the glass rod, and failed, but was obstinate in my belief I could write on it invisibly with something. I had an inspiration and said,

“I still think I can beat it if you’ll let me try again”. “Impossible!” they said. “We’ve tried everything”.

I said, “Well, let me try once more. Bring me a clean rubber stamp and some vaseline”.