When I was an astronomy graduate student, there was a shattered glass photographic plate, meticulously glued back together and framed on the wall next to the measuring engine in the plate library of the observatory. (It was, of course, now useless for providing careful measurements of stellar positions or brightnesses). The library contained thousands of plates, each individually stored in brown paper envelopes with their relevant information neatly inked on them in the spidery hands of generations of astronomers, going back for almost a century. It was a permanent record of raw data for millions of stars, archived for the attention of any researcher who might want to compare them with modern observations. It was also an irreplaceable and irreproducible resource because the main ingredient in their composition was lost forever to us: the moment in the past when they were exposed.

On the frame of the mounted, broken plate was affixed a little brass plaque on which was carefully engraved "We have the only janitor in the world with a PhD in astronomy."