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For many applications, a narrow absorption band may be desirable.

Later researches, especially those of S. P. Langley with the spectro-bolometer on the infra-red spectrum of sunlight, demonstrated the existence of marked absorption bands, some of which are due to water vapour.

And the study of the absorption bands in the spectrum of the living leaf becomes more intelligible in the light of these conclusions.

The bodies which exhibit the first kind are those which possess strong absorption bands, of which only one remains appreciable after great dilution.

Only very thin layers are sufficiently transparent to show the dispersion near or within an absorption band, and a large refracting angle is not required, the dispersion usually being very considerable.