Untitled Another way of obtaining considerable dispersion is by using a _diffraction grating_ instead of a prism. This consists essentially of a piece of glass on which lines are ruled by a diamond point. When the lines are sufficiently close together they split up light falling on them into its constituents and produce a spectrum. The modern diffraction grating is a truly wonderful piece of work. It contains several thousands of lines to the inch, and these lines have to be spaced with the greatest accuracy. But in this instrument, again, there is a considerable loss of light. We have said that every substance has its own distinctive spectrum, and it might be thought that, when a list of the spectra of different substances has been prepared, spectrum analysis would become perfectly straightforward. In practice, however, things are not quite so simple. The spectrum emitted by a substance is influenced by a variety of conditions. The pressure, the temperature, the state of motion of the
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