Abstract
ALL workers with the spectroscope must have felt the great inconvenience arising from the employment of numberless different scales in the mapping of spectra. It is to be hoped that at some future time there will be more uniformity, and that authors, when publishing original memoirs, will reduce their measurements to a definite and recognised system. It is clear that such a method must be perfectly independent of the spectroscope and its con-comitant parts; the position of each line can therefore only be expressed by its colour, or, in other words, by the length of the wave of light which produces this colour. Dispersion spectra, obtained by the use of prisms of different materials, vary greatly in the relative breadth of the respective colours; thus in the spectrum from crown-glass the red end is larger and the blue end shorter than in the spectra obtained from flint-glass, carbonic disulphide, and by diffraction. It is therefore necessary in spectroscopic researches to record the positions of numerous well-known lines as observed in the instrument that is used. In a diffraction spectrum, however, the position of the lines is dependent solely on their colour, and is precisely the same by whatever method the spectrum is obtained. For the results of different observers to be I accurately comparable, the readings obtained by dispersion must either be expressed in wave-lengths, or the spectra must be obtained by diffraction. The wave-lengths of the Fraunhofer lines of the sun have been accurately determined by several observers. The author has adopted as the basis of his work the measurements made by Angström, as these appear to exceed in accuracy all similar measurements at our disposal. When the wave-lengths of a number of lines are known, it is easy to calculate the wave-lengths of the lines of any new spectrum, either by the interpolation formula given by W. Gibbs Phil. Mag. [4] xl.157) or by the method of graphical inter-polation, both of which methods are explained in the volume before us; all that is required is to have the wavelengths of two known lines, between which the line to be measured falls. By the aid of Angstrom's measurements the author has reduced the measurements of the bright lines of all the elements whose spectra have been carefully investigated, and also of air lines as mapped by Thaler, Huggins, and Plucker. These tables will therefore assist materially in the work of reduction, by serving as landmarks from which to calculate the wave-lengths of new lines. The attention that the author has bestowed on this work is the best guarantee of the accuracy of the numbers given. In the lithographic plates at the end of the tables, a drawing of the spectrum of each element is given on the plan proposed by Bunsen, in which the intensity of a bright line is indicated by the height of the line representing it; a chromo-lithograph is given of the double spectra of nitrogen, sulphur, and carbon, and another plate, showing two spectra obtained by Wülner from aluminium, and three from hydrogen at different powers. Dr. Watts is deserving of the best thanks of all those interested in spectroscopic work, for it is to be hoped that his “Index of Spectra” may contribute to the adoption of a uniform scale of measurement, and thus facilitate the advance of the science.
Index of Spectra.
By W. M. Watts (London: Henry Gillman.)
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P., A. Index of Spectra . Nature 5, 442 (1872). https://doi.org/10.1038/005442a0
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DOI: https://doi.org/10.1038/005442a0