xxi 



much attention from physicists, from the fact that the determination of 

 the condiicting-power is made independently of their radiating-power. 



Angstrom's first treatise of any length on optics was that ' On Recti- 

 linear Polarization and on the Double Eefraction of Crystals with Three 

 Oblique Axes/ by which he has contributed to the explanation of the 

 optical properties of those crystals. "With this Memoir is connected his 

 note ' On the Molecular Constants of Monoclinic Crystals.' He also 

 wrote on the question of the principal properties of the plane of polari- 

 zation, and made experiments on the capacity of absorption of chloro- 

 phyl. 



Angstrom's important work, ' Optical Eesearches,' was presented to 

 the Academy of Sciences of Stockholm in 1853. In this work he has 

 shown that the spectrum of the electric spark is formed by the super- 

 position of two spectra — one of them due to the metallic pole, the other 

 to the medium through which the spark passes. Following up the obser- 

 vations made by Wheatstone and Masson, he found that the spectrum 

 obtained from an alloy of two metals chemically combined with one 

 another contains the spectrum of each of the two metals. 



o 



In the same memoir. Angstrom propounds the theory that the only 

 luminous rays which a vapour or gas can absorb are those which it emits 

 when highly incandescent. It was apropos of this theory that Sir Edward 

 Sabine, in his discourse to the Eoyal Society on the occasion of the election 



o 



of M. Angstrom as a Eoreign Member, remarked that the memoir contains 

 the fundamental principles of all subsequent progress in spectrum- 

 analysis. 



The continuation of the spectrum researches, published at short in- 



o 



tervals by Angstrom between the years 1860 and 1870, are well kno\\TQ to 

 physicists. These are : — in 1861, a memoir on the lines of the solar 

 spectrum ; in 1863, a new determination of the lengths of the luminous 



o 



waves ; and in 1865, a memoir, published jointly by Angstrom and 

 Thalen, on the violet part of the solar spectrum, which paved the way for 



o 



the great work which Angstrom published in 1868, ' Eesearches on the 

 Solar Spectrum,' containing the determinations, founded on exact mea- 

 surements, of the length of waves for the different lines of Eraunhofer. 

 His intention was also to treat in this manner the question of the double 

 spectra of bodies ; but death interrupted his labours, leaving him only 

 time to discuss, in the ' Comptes Eendus' of the Erench Academy of 

 Sciences, some ideas put forth by M. "Wiillner. 



Another work of the same sort, on the Spectra of the Metalloids, had 



o 



been begun by Angstrom some years previously, and was partly printed 

 before his death. Its publication was completed in 1875 by Thalen, in 

 the Transactions of the Society of Sciences of Epsala. A note of his 

 on the Spectrum of the Aurora Borealis, which he was the first to examine. 



