On photographic plates no bigger than a stick of chewing gum, astronomers can look at a spectrum so tiny that it must be viewed through a jeweler's eyepiece and tell what kind of atoms are dancing in a star trillions of miles away. Thus has astronomy advanced since Galileo first glimpsed the four big satellites of Jupiter and wondered what they were. The mod ern art of splitting up light into its com ponent colors, which disclose the chemical nature of the source, depends on a little thing called the diffraction grating....
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