Phenomena observed in the Absorption-spectrum of Didymium. 1 7? 



difference between devising theories in the cabinet, or even trying 

 isolated experiments, and actually carrying out those methods on a 

 large scale by the aid of an organized staff. I concede botn merits 

 to Dr. Siemens; and if I have urged my arguments forcibly as to 

 the independence of the English school of electricians of that of 

 Germany, I beg Dr. Siemens to believe that I have done so from no 

 desire to diminish his claims, which indeed I could not do, but only 

 to vindicate myself from what I felt to be a very unmerited suspi- 

 cion, that of having wilfully omitted to mention his discoveries. 



XXIII. On the Phenomena observed in the Absorption-spectrum 

 of Didymium, By R. Bunsen*. 



[With Two Plates.] 



IN a paper which Professor Bahr and I published together, 

 "Upon the Compounds of Erbium and Yttrium" f, we showed 

 that slight differences were observed in the absorption-spectrum 

 of sulphate of didymium, according as the light was allowed to 

 pass through a crystal or through a solution of the salt. Since that 

 time I have found that the erbium- and the didymium-spectrum 

 undergo alteration if polarized light be employed and either 

 the ordinary or the extraordinary ray be allowed to pass through 

 the crystal. I have also found that whilst, when spectroscopes 

 with one prism and with a telescope of moderate power are em- 

 ployed, the spectra of the various didymium compounds do not 

 show any difference, yet most undoubted differences are noticed 

 when more powerful instruments are used. 



The alterations which the absorption-spectra exhibit under 

 these circumstances form the subject of the present communi- 

 cation. 



In the following experiments two of SteinheiPs spectroscopes 

 were used. In the one, which I call the smaller, was placed one 

 flint-glass prism having a refracting angle of 60°, and a refract- 

 ing surface of 30 millims. in diameter, whilst the telescope had 

 a magnifying power of 8 : in the other, which I call the larger, 

 were four large prisms of flint glass, one of which had a refract- 

 ing angle of 60°, and the other three each of 45° ; the magnify- 

 ing power of the telescope was "40. The observations with the 

 smaller instrument are reduced to the same scale of measurement 

 as that used in the Table figured in the Philosophical Magazine, 

 S. 4. vol. xxvi. p. 241. With the larger instrument the scale 

 was employed which Kirchhoff adopted in his researches on the 

 solar spectrum. 



* Translated and communicated by Professor Roscoe, from Poggen- 

 dorff's Annalen, vol. cxxviii. p. 100. 



\ Ann. der Chem. und Pharrn, vol. cxxxvii. p. 1. 



Phil. Mag. S. 4. Vol. 32. No. 215. Sept. 1866. N 



