Abstract
I. TH? Royal Institution seems a peculiarly fit place to deliver lectures on this subject, because it was while he was professor here 120 years ago that Thomas Young, the great advocate of the wave theory of light, showed how to estimate the wave-lengths of the different parts of the spectrum, and by so doing laid the foundations of spectroscopy as a quantitative science. His determinations of the wave-lengths in the visible spectrum were based on Newton's observations of the colours of thin plates. He also explained the principle of the diffraction grating, and by experiments based on the method of Newton's rings he showed that the actinic or ultra-violet rays had shorter wave-lengths than those in the visible. The wavelengths of the visible spectrum extend from a little below 4000 to a little above 7000 Angstrom 2 units, or, roughly, over about an octave. On the infra-red side we have, first, the invisible rays, often referred to as radiant heat, which contain the major part of the energy in the solar spectrum and a greater proportion still of the energy radiated from bodies at a lower temperature. Beyond these we have the long electromagnetic waves of the type we are familiar with in wireless telegraphy. This side of the spectrum extends to waves of infinite length or of zero frequency.
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RICHARDSON, O. The Disappearing Gap in the Spectrum. Nature 111, 118–121 (1923). https://doi.org/10.1038/111118a0
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DOI: https://doi.org/10.1038/111118a0