34: Rubens and Trowbridge — Dispersion and Absorption 



22'3/Lt, and with the help of these points, together with the 

 above mentioned earlier data, the five constants of the Ketteler- 

 Helmholtz equation 



b 2 + 



m , 



m„ 



A a -X 



have been calculated.* In the tirst part of this communica- 

 tion we have limited ourselves to the comparison of the results 

 obtained by means of this formula, with the dispersion of rock 

 salt and sylvine for certain points in the large field between 

 \ = 8'86/Lt and X — 20'6/ul for rock salt and X = 7'06/x. and 

 X = 20'6yu- for sylvine, for which up to the present time no 

 experimental determinations have been made. 



We employed a method based on the comparison of the 

 grating spectrum and the prism spectrum which was similar to 

 Langley'sf method and which differed from it in only one 

 point. While in Langley's arrangement the dispersion of the 

 rays was obtained first from the grating and then from the 

 prism under examination, in our arrangement the dispersion 

 was brought about in the inverse order. This change was of 

 little importance and was effected by us only because in that 

 way we were able to set up our apparatus more conveniently. 



£ <C-:-:- s '-'- 



^i 



•# 



___-_^a/[ 



The arrangement of our apparatus is given in figure 1. In 

 it a represents the source of heat (Linnemann zircon burner) 



* H. Kubens and E. F. Nichols, Wied. Ann., 1, p. 454, 1897. 



f Langley, this Journal, xxvii, 169; Wied. Ann., xxii, p. 598, 1884. 



