Spectrum analysis in its application to terrestrial substances, and the physical constitution of the heavenly bodies / Familiarly explained by the late H. Schellen. Trans. from the 3d enl. and revised German edition by Jane and Caroline Lassell.

Heinrich Schellen

Date:: 1885

Credit: Spectrum analysis in its application to terrestrial substances, and the physical constitution of the heavenly bodies / Familiarly explained by the late H. Schellen. Trans. from the 3d enl. and revised German edition by Jane and Caroline Lassell. Source: Wellcome Collection.

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$in the flint-glass spectrum than in the water spectrum, and this difference is more apparent than in the crown- glass spectrum. It would be, therefore, an error to take for granted that the distances between individual dark lines in the spectrum* change in exactly the same proportion as the entire length of the spectrum. Even if the dispersive power of a sub- stance is known for the extreme ra}'s, or for the lines B and H, the amount of Separation between the interv’ening lines of the spectrum cannot be deduced from it; the relative position of these lines must be specially ascertained for each refracting substance. An accurate knowledge of the particular conditions of the spectrum apparatus employed must therefore be acquired by every observer before any value can be given to the results of observations made with it; he must become familiär with the precise places of all the chief lines and groups of lines seen in the solar spectrum, so that in the examination of an\- particular line, whether in the spectrum of a terrestrial substance or of a heavenly body, he may know, at least approximately, to which Fraunhofer line it lies nearest. 22. Indices of Refraction a.\d Amouxt of Dispersion for THE MOST Prominent Fraunhofer Fines. Before the discovery of the Fraunhofer lines, the indices of refraction for the various coloured rays could only be approximatel}' ascertained by estimations from the yellow— the central portion—of the spectrum ; but at present the dark lines aftbrd an easy means of determining the index of refraction for every part in the spectrum. Many of these measLires were made with great exactness by Fraunhofer, [When the prisms are formed of dift'erent materials.] *$