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Blink Comparator

In the days before computers, when the possibility of intellectual achievement presupposed infinite tolerance for drudgery, the blink comparator was a machine used in astronomy.

Two photographic plates – negative images of successive telescopic views of the same region of space – were inserted into the machine.

An astronomer would sit before the machine, watching carefully – watching with absolute attention – while the machine flicked back and forth between the two images. The rapid alternation of the images, like frames in a movie, gave any change the appearance of motion. This was much easier than looking back and forth; but still, it must have been very hard. No one, not the fussiest and most fanatical director who ever worked, has ever watched a moment of film so intently as did those astronomers who, once upon a time, watched the dots and blots cycle in a blink comparator.

It was by use of the blink comparator that Pluto was discovered. The demotion of Pluto makes the workings of the blink comparator less historically important; yet it makes them more wonderful. In the last decade we have found that Pluto is only one among many dwarf planets circling the periphery of the solar system. Again, we have learned this in the last decade. But it was in 1930 that Clyde Tombaugh discovered Pluto with a blink comparator – a discovery made 80 years ahead of its time by the surpassingly skillful use of a surpassingly difficult instrument.

Obsolete instruments often live on in metaphors. (Only amateur astronomers put eye to telescope.) The blink comparator deserves such a metaphorical legacy. What does the mind lack more than this device? In reaching for memory we lose perception; in attending to perception we let go of memory – though each is useless, except when set in contrast with the other. We must plod to present choices to our judgment – we concentrate on one choice only by neglecting the other; in reconsidering the first, we lose the second.

But the blink comparator is not only a metaphor of aspiration. There are moments in life that come as if through such a device: moments when the past somehow overlays the present, when you are at once who you are, and who you were, and some third who sees both at once; moments when the present is weaker and less believable than the past. Such a moment is mourning, when you are at once still the person you were before your loss, and the person you must become now, and some third who sees and guides the change.

These the moments when the sense of mystery in life is strongest: when something new and nameless is seen to move, and in the dark of the room the astronomer knows the poet’s wild surmise.