"My feeling is that the chance of our surviving into the twenty-first century as working civilization is less than fifty percent but greater than zero."
- Isaac Asimov

A blink-mike provides accommodation for two photographic plates taken of the same section of sky, but at different times. These plates are carefully juxtaposed and the operator may alternately focus his vision, through the eyepiece, first upon one and then upon the other, by means of a shutter. If the plates are identical, the operation of the shutter reveals nothing, but if one of the dots on the second plate differs from the position it occupied on the first, it will call attention to itself by seeming to jump back and forth as the shutter is manipulated.

Compare this to the actual device used to discover Pluto. It was invented in 1904 by physicist Carl Pulfrich at Carl Zeiss AG.

(The Blink Comnparator used to find Pluto)

The photographic plates Tombaugh was comparing with this machine were 36 x 43 centimeters (14 x 17 inches), and were long exposures taken with a telescopic camera that sported a powerful 33-centimeter (13-inch) diameter lens. Tombaugh took exposures at night, covering areas of the sky where Percival Lowell had predicted years earlier that a planet must be lurking. He then developed those plates, and during the day compared them, spending weeks and months searching vast depths of space looking for something moving among the thousands of stars exposed on those plates. In February 1930, finally, he found something that he could not explain away as a nearby asteroid or some other form of space debris. It moved too slowly to be an asteroid. If it was a planet, it was farther than Neptune, just as Lowell predicted.

Tombaugh operated the instrument, built by the Zeiss company and popularly called a “blink comparator,” by rotating a small dial that flipped a mirror back and forth between the beams from two microscopes. Blinking between two plates taken on January 23 and 29, 1930, he found something. The below animation is flipping between the two plates. Can you find it? If you're having trouble spotting Pluto, take a look at the next animation with arrows marking the elusive object. It’s not an easy job, to be sure. The time exposures do not always yield the same brightness, and the orientation of the two fields is not exact.