We can appreciate the beauty of the Grand Canyon only because of our eyes' selective viewing, according to an American engineer who has used electronic instruments to measure the canyon's colours. Ronald Henry of the University of Southern California, Los Angeles, says that although the rocks of the canyon seem brightly coloured at a distance, instruments show them to to have a spectrum which looks washed-out purple.
According to Henry, the eye and brain combine to give distant rocks the colours they would have if they were closer to the observer. He does not know the exact mechanism, however.
Henry began studying the Grand Canyon to assess how air pollution affected visibility, an important issue in the scenic southwestern US. He asked observers to identify the colours they saw, and compared the results with electronic measurements of the light intensity at different wavelengths. Henry found that for nearby rocks the ...
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