ICELAND SPAR • DICTIONARY ONE.COM • Definition of Iceland spar

... an impression of four lines in the spectrum of Vega. Then Huggins attacked the subject again in 1876, when the 18-inch speculum of the Royal Society had come into his possession, using prisms of Iceland spar and lenses of rock crystal; and this time with better success. A photograph of the spectrum of Vega showed seven strong lines.[1415] Still he was not satisfied. He waited and worked for three years longer. At length, on December ...

— A Popular History of Astronomy During the Nineteenth Century - Fourth Edition • Agnes M. (Agnes Mary) Clerke