“Consider the case of the American astronomer Percival Lowell. In 1896, while peering through his twenty-four-inch-diameter telescope, Lowell observed what he firmly believed to be canals on the surface of the planet Venus. Lowell was already famous for spotting canals on Mars - proof, he convinced himself (and the lay public), of the existence of inteligent Martians, or at least Martians with shovels. His observations in 1896 led him to conclude that Venus was likewise inhabited. Where Lowell saw canals, however, fellow astronomers saw only a blurry white sphere; they could not replicate his observations. Neither could Lowell himself, until at last, in 1903, he saw the canals once more and even managed to draw a picture of them. Lowell died in 1916; the canals were never seen again.
Only recently have scientists figured out what he saw. A team of ophthalmologists, examining Lowell’s notes, determined that the astronomer had made the aperture of his telescope so small that he had unwittingly turned it into a mirror: what he thought were canals were, in fact, the reflected shadows of the blood vessels in his retina. He had stamped his eye on the cosmos and mistaken it for the real thing. It is every scientist’s nightmare: to mistake inside for outside; to confuse the order one sees, or wishes to see, or even, as in Lowell’s case, one actually does see, with what is actually there.” — Out of Eden by Alan Burdick