While some substances emitted light slowly in the dark after being exposed to daylight, others glowed only while they were being illuminated. This was fluorescence (after the mineral fluorite, which often showed it). This strange luminosity had been originally discovered as early as the sixteenth century, when it was found that if a slanting beam of light was directed through tinctures of certain woods, a shimmering color might appear in its path – Newton had attributed this to ‘internal reflection.’ My father liked to demonstrate it with quinine water – tonic – which showed a faint blue in daylight and a brilliant turquoise in ultraviolet light. But whether a substance was fluorescent or phosphorescent (many were both), it required blue or violet light or daylight (which was rich in light of all wavelengths) to elicit the luminescence – red light was of no use whatever. The most effective illumination, indeed, was invisible – the ultraviolet light that lay beyond the violet end of the spectrum.
My own first experiences of fluorescence occurred with the ultraviolet lamp my father kept in the surgery – an old mercury vapor lamp with a metal reflector, which emitted a dim bluish violet light and an invisible blaze of ultraviolet. It was used to diagnose some skin diseases (certain fungi fluoresced in its light) and to treat others – though my brothers also used it for tanning.
These invisible ultraviolet rays were quite dangerous – one could be severely burned if exposed too long, and one had to wear special goggles like an aviator’s, all leather and wool, with thick lenses made of a special glass that blocked most of the ultraviolet (much of the visible, too). Even with goggles, one had to avoid looking directly at the lamp, otherwise a strange, unfocused glow appeared, due to the fluorescence of one’s eyeballs. One could see, looking at other people in the ultraviolet light, how their teeth and eyes glowed a brilliant white.
Uncle Abe’s house, a short walk from ours, was a magical place, filled with all sorts of apparatus: Geissler tubes, electromagnets, electric machines and motors, batteries, dynamos, coils of wire, X-ray tubes, Geiger counters and phosphorescent screens, and a variety of telescopes, many of which he had built with his own hands. He would take me up to his attic laboratory, on weekends especially, and once he had satisfied himself that I could handle the apparatus, he gave me the run of his phosphors and fluorescent materials, as well as the little handheld Wood’s UV lamp he used (this was much easier to deal with than the old mercury vapor UV lamp we had at home).
Abe had racks and racks of phosphors in his attic, which he would blend like an artist with his palette – the deep blue of calcium tungstate, the paler blue of magnesium tungstate, the red of yttrium compounds. Like phosphorescence, fluorescence could often be induced by ‘doping’, adding activators of various sorts, and this was one of Abe’s chief research interests, for fluorescent lights were just beginning to come into their own, and subtle phosphors were needed to produce a visible light that was soft and warm and agreeable to the eye.[54] Abe was especially attracted to the very pure and delicate colors which could be made if one added various rare earths as activators – europia, erbia, terbia. Their presence in certain minerals, he told me, even in minute quantities, lent these minerals their special fluorescence.
But there were also substances that would fluoresce even when absolutely pure, and here uranium salts (or, properly speaking, uranyl salts) were preeminent. Even if one dissolved uranyl salts in water, the solutions would be fluorescent – one part in a million was sufficient. The fluorescence could also be transferred to glass, and uranium glass or ‘canary glass’ had been very popular in Victorian and Edwardian houses (it was this which so fascinated me in the stained glass in our front door). Canary glass transmitted yellow light and was usually yellow to look through, but fluoresced a brilliant emerald green under the impact of the shorter wavelengths in daylight, so it would often appear to shimmer, shifting between green and yellow depending on the angle of illumination. And though the stained glass in our front door had been shattered by a bomb blast during the Blitz (it was replaced by an unpleasant, bobbly white glass), its colors, intensified by nostalgia perhaps, still remained preternaturally vivid in my memory – especially now that Uncle Abe had explained its secret to me.[55]
Though Abe had expended much effort on the development of luminous paints, and later on phosphors for cathode-ray tubes, his central interest, like Dave’s, was in the challenge of illumination. The hope he had nourished, from early on, was that it might be possible to develop a form of cold light as efficient, as pleasant, as tractable, as hot light. Thus while Uncle Tungsten’s thoughts were fixed on incandescence, it was clear to Uncle Abe from the start that no really powerful cold light could be made without electricity, and that electroluminescence would have to be the key. That rarefied gases and vapors glowed when electrically charged had been known since the seventeenth century, when it was observed that the mercury in a barometer could become electrified by friction against the glass, and this would set up a beautiful bluish glow in the rarefied mercury vapor in the near vacuum above.[56]
Using the powerful discharges from the induction coils invented in the 1850s, it was found that a long column of mercury vapor could be set glowing (Alexandre-Edmond Becquerel suggested, early on, that coating the discharge tube with a fluorescent substance might make it more suitable for illumination). But when mercury-vapor lamps were introduced, for special purposes, in 1901, they were dangerous and unreliable, and their light – in the absence of a fluorescent coating – was too blue to allow domestic use. Attempts to coat such tubes with fluorescent powders before the First World War collapsed before a multitude of problems. Other gases and vapors, meanwhile, were being tried: carbon dioxide gave a white light, argon a bluish light, helium a yellow light, and neon, of course, a crimson light. Neon tubes for advertising became common in London by the 1920s, but it was only in the late 1930s that fluorescent tubes (using a mixture of mercury vapor with an inert gas) started to become a commercial possibility, a development in which Abe played a considerable part.
Uncle Dave, to show he was not bigoted, had a fluorescent light installed in his factory, and the two brothers, who had seen the tussle of gas and electricity in their youth, would sometimes argue about the respective merits and drawbacks of incandescent and fluorescent bulbs. Abe would say that filament bulbs would go the way of the gas mantle, Dave that fluorescents would always be bulky, never a match for the ease and cheapness of bulbs. (Both would have been surprised to find, fifty years later, that while fluorescents had evolved in all sorts of ways, filament bulbs remained as popular as ever, and that they coexisted in a comfortable and fraternal relationship.)
The more Uncle Abe showed me, the more mysterious the whole thing became. I understood a certain amount about light: that colors were how we saw different frequencies or wave-lengths; and that the color of objects came from the way they absorbed or transmitted light, obstructing some frequencies, letting others through. I understood that black substances absorbed all the light, letting nothing through; and that with metals and mirrors it was the opposite – the wave front of light particles, as I imagined it, hit the mirror like a rubber ball and was reflected in a sort of instant bounce.
But none of these notions was helpful when one came to the phenomena of fluorescence and phosphorescence, for here one could shine an invisible light, a ‘black’ light, on something and it would glow white or red or green or yellow, emitting a light of its own, a frequency of light not present in the illuminant.
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Equally important were cathode-ray tubes, which were now being developed for television. Abe himself had one of the original television sets of the 1930s, a huge, bulky thing with a tiny circular screen. Its tube, he said, was not much different from the cathode-ray tubes that Crookes had developed in the 1870s, except that its face was coated with a suitable phosphor.
Cathode-ray tubes in use for medical or electronic apparatus were often coated with zinc silicate, willemite, which emitted a brilliant green light when bombarded, but for television one needed phosphors that would give a clear, white light – and if color television was to be developed, one would need three separate phosphors with exactly the right balance of color emissions, like the three pigments in color photography. The old dopants used in luminous paints were quite unsuitable for this; much more delicate and precise colors were needed.
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Uncle Abe also showed me other types of cold light. One could take various crystals – like uranyl nitrate crystals, or even ordinary cane sugar – and crush them, with a mortar and pestle, or between two test tubes (or even one’s teeth), cracking the crystals against one another – this would cause them to glow. This phenomenon, called triboluminescence, was recognized even in the eighteenth century, when Father Giambattista Beccaria recorded:
You may, when in the dark frighten simple people only by chewing lumps of sugar, and, in the meantime, keeping your mouth open, which will appear to them as if full of fire; to this add, that the light from sugar is the more copious in proportion as the sugar is purer.
Even crystallization could cause luminescence; Abe suggested that I make a saturated solution of strontium bromate and then let it cool slowly in the dark – at first nothing happened, and then I began to see scintillations, little flashes of light, as jagged crystals formed on the bottom of the flask.
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The same phenomenon, I read, had been used ingeniously to make self-luminous buoys – these were encircled by rings of strong glass tubing containing mercury under reduced pressure, which would be swirled against the glass and electrified by the motion of the waves.