“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”.
The large, smooth, clean rubber stamp was brought. I smeared it with vaseline, then wiped it carefully with a cloth until it made no visible grease mark on paper. Then I pressed it down firmly on the spy-proof paper, taking care not to let it slip sideways.
“Can you discover any writing here?” I asked.
They studied it by reflected light and by polarized light and said, “Nothing here”.
“Now let’s look at it with the ultraviolet”, I said. We took it into the cabinet and held it in front of my black window. In brilliant blue letters, as if the nice clean rubber stamp had just been pressed on its own ink pad, stood the words:
NO SECRET
WRITING HERE
Professor Wood, now in mufti and with a discharge paper indorsed “Honest and Faithful”, began traveling again over the battlefields in the war zone to see whether any signaling apparatus used by the Germans could be picked up. He started off in an Army Cadillac with Lieutenant Winchester and Dike of the American Embassy.
They went through trenches and down into dugouts, but the only pieces of German optical apparatus they discovered were primitive signal lamps for giving a narrow beam, made out of old brass shell cases, with a candle at the bottom and a narrow slit in the side.
Of Wood’s final days in Europe after the war, he writes:
Before leaving Paris I was asked to give a lecture with demonstrations at the Sorbonne. The war was over, and “Now it could be told”. The show came off on May 18 in the large lecture hall, before an audience of two hundred or more, composed of physicists and army officers, some with their ladies. With the room darkened and a very powerful ultraviolet lamp, I flooded the audience with what the French had named Lumière Wood, causing teeth and eyes to phosphoresce brilliantly, and various textiles to shine with subdued colors. A lady’s dress in the center of the hall glowed with a brilliant scarlet color, attracting much attention. Everyone was looking at the glowing teeth and eyes of their neighbors, and a wave of laughter swept the hall when I explained that false teeth appeared as black as charcoal in the light. With a flash telescope, I demonstrated the narrow beam of light, and the lecture closed with “Vive la France”, rapidly executed with the spot of light on the wall in Morse code, which was read by a sufficient number of officers to cause applause.
Chapter Fifteen.
The Woods Cover the World — The Barn Spectroscope Moves to a Palace — And Pussycat Loses Her Job
Wood’ s fiftieth birthday had occurred in the year of the armistice. The years which followed saw him entering an era of speeded-up activities, scientific and social, which would have left most younger men out of breath and panting.
He detests the word “social” and “society”, but this petulance doesn’t alter the fact that he and the whole family have always loved parties and gaiety. By 1918 they had become international cosmopolites, shuttling in the summertime from great country houses in England to Paris and Brittany, to St.-Jean-de-Luz, to St.-Moritz in winter, playing with the most dazzling playboys and playgirls of the period as well as consorting with fellow-celebrities in Wood’s own world of science.
A complete record of the Wood family’s ocean trips, parties, visits to Aix-les-Bains, Baden-Baden, Biarritz, Venice, and the Lido during the twenties and thirties would give the false impression that Wood himself was an international playboy. Yet it was during these same years that he conducted many of his most important researches and made some of his greatest contributions to science. The man is definitely a hyperkinetic, yet never burns out his fuses.
Wood was no sooner out of the army and back in Baltimore than he took up his work with sodium where he had left off before the war. In 1919 he announced the discovery that thin films of metallic sodium and potassium, condensed on the inner surface of fused quartz bulbs at liquid-air temperature, while so opaque to light that the sun was invisible through them, were transparent to the entire range of the ultraviolet as far as wave length 2,000. The discovery was of considerable importance in connection with the new theory of the optical properties of metals. At the same time Wood became interested in the mystery of the hydrogen spectrum — the fact that while all terrestrial sources of luminous hydrogen gave spectra with only eight of the so-called Balmer series of lines, the spectra of the sun’s chromosphere and many stars showed thirty-three members of the series. The result of his investigation is such a good example of how research in pure science can have immediate practical importance that I let him tell it himself.