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We now jump from mathematics to physics and from 1637 to 1895, when the German scientist Wilhelm Röntgen discovered a new type of radiation. Röntgen, who wasn’t sure just what he had come across, named his find in German X-Strahlen, using the algebraic symbol for something unknown. Although X-Strahlen was eventually replaced in German by Röntgenstrahlen, “X-ray” has remained our English word and has also contributed, with the help of the term “X-ray vision,” to x’s ability to evoke the uncanny.
From here we backtrack slightly to 1883, when Robert Louis Stevenson’s classic children’s novel, “Treasure Island,” with its tale of buried pirate loot, popularized the expression “X marks the spot,” an allusion to the letter x’s use to indicate precise locations on detailed maps. This x has nothing to do with algebra. It is used in this way because its two intersecting lines create an easily visible point where they meet — and yet by a curious coincidence, this, too, associates x with secrecy and mystery.
Finally, we have the equally unrelated idiom “to x out,” dating from the pre-Typex days of typewriters, when printing a row of x’s over a typed line was the standard way of erasing. (X’s may have been picked for the job because they’re good at covering the shapes of other letters.)
This typewriter x was probably the inspiration for the x on your computer screen that shuts down boxes, pop-ups and programs, which is what the phrase “x-ing out” calls to mind today, as well as the source of the scrawled X with which we blot out illustrations we don’t wish to be seen, and the term “X-rated.” And so, once again, we find “x” connected with what is hidden.
Indeed, so established was the letter x as a symbol of the unknown long before the 1930s, one suspects that rather than being called “Doctor X” because his name was Xavier, Lee Tracy’s physician-sleuth was given the name Xavier because the film’s scriptwriters wished to call him “Doctor X.” Perhaps, now that we know his real name, Prisoner X should be called “Prisoner ZY.” Or as Descartes might have put it, letting “m” stand for Mossad, m(zy)=x2.
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