On the spectra of the alkaline earth fluorides and their relation to each other

It is well known that the limits of the series and the separation of doublets and triplets, which occur in the line spectra of chemically related elements, are intimately connected with the atomic weights of the elements involved. It seemed reasonable to suppose that numerical relations might also exist in the case of band spectra, and the present paper gives an account of an investigation undertaken to determine possible relations among the band spectra of similar compounds of different elements. Previous work by other observers suggested the fluorides of the alkaline earth metals as a suitable group of compounds for the purpose in view. In the first instance a more complete survey of the spectra of these compounds was made, and several bands in addition to those previously described have been observed. These new bands have proved of considerable service in the identification of corresponding bands in the different spectra, and details relating to them are accordingly included in the paper. The beautiful bands of CaF2, SrF2, and BaF2 were first observed in the flame spectra by Mitscherlich as far back as 1862, but no systematic measurements were made until 1905, when observations of the bands of these compounds in the visible region were made by Fabry with a spectroscope of not very large dispersion. Measurements of the same spectra from photographic plates were first made by Rosch and further measures were subsequently made by Leopold in the case of SrF2, and by George in the case of BaF2.