Blink comparator

In the days before computers, when the possibility of intellectual accomplishment presupposed infinite tolerance for drudgery, the blink comparator was a device used in astronomy. It worked as follows: two photographic plates—negative images of separate telescopic views of the same region of space—were inserted into a machine.

An astronomer would sit before the machine, watching carefully—watching with inhuman attention—while the machine flicked back and forth between the two images. The rapid flashing of a sequence images, like movie stills gave any change the appearance of motion. This was much easier than looking back and forth; but still, it must have been very hard. Perhaps 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 a pair of patterns of dots and blots cycle in a blink comparator.

It was by this means 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 almost 80 years ago, in 1930, that Clyde Tombaugh discovered Pluto at 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 faculty? 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. We must plod to present comparisons to our judgment—we can concentrate on one alternative only by neglecting another; and in recovering 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 to weigh them; moments when the present is weaker and less believable than the past—as in mourning, when you are at once still the person you were before the 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.