250 M. G. Kirchhoff on the History of Spectrum Analysis 



the plane (10 0). It seemed, moreover, that this section is per- 

 pendicular to the first mean line, although the points of the 

 optic axes were not in the field of view. In consequence the 

 crystals would be positive. The double refraction is weak. 



The following* Table contains the optical orientation of the 

 salts isomorphous with the sulphate of potassium which I have 

 hitherto had the opportunity of examining : — 





SO 4 



SeO 4 



TeO 4 



CrO 4 



Am 



fcac 



+ 



6ac 







K 



ac6 



+ 





ac6 



+ 



ac6 



Th 



6ac 



+ 









It will be seen from this Table that, in its optical properties, 

 the sulphate of thallium accords with the ammonium, and not 

 with the potassium salt. It would not only be interesting to 

 fill up some of the blanks of the above Table, but also of import- 

 ance to investigate at least the sulphates of the new elements 

 caesium and rubidium, which belong to the same isomorphous 

 group. Another desirable object of investigation would be the 

 influence exercised on the optical orientation by the substitution 

 of alcohol radicals in the sulphate of ammonium, provided 

 these new combinations are isomorphous with their original type, 

 which, as far as I am aware, has hardly been proved by measure- 

 ments even in the case of the simplest of these bodies. 



XXXVI. Contributions towards the History of Spectrum Analysis 

 and of the Analysis of the Solar Atmosphere. By G. Kirch- 

 hoff*. 



IN my l Researches on the Solar Spectrum and the Spectra of 

 the Chemical Elements' f, I made a few short historical 

 remarks concerning earlier investigations upon the same subject. 

 In these remarks I have passed over certain publications in 

 silence — in some cases because I was unacquainted with them, 

 in others because they appeared to me to possess no special 

 interest in relation to the history of the discoveries in question. 

 Having become aware of the existence of the former class, and 

 seeing that more weight has been considered to attach to the 



* Communicated by Professor Roscoe. 



t Published by MacMillan and Co., Cambridge and London, 1862. 



