in the Velocity of Propagation of Light in Water. 281 



We have the continuous proportion 



A Li : A D : A Th = sin 1° 50' 49" : sin 1° 37' 16" : sin 1° 28' 21". 



Since now A D is known, we thence get 



A Li = 0-0006708, A Th = 0-0005348. 



If, further, we remember that, according to Fraunhofer's 

 measurements*, 



A Li = 0-0006708, 



A B = 0-00068781 



A c = 0-0006564 J 



A D = 0-00058881 A nm ,, Q , Q 

 y A Th = 0-0005348, 



A E =0-0004843 J 



we see that the red band of lithium falls between B and C, and 

 that the homogeneous green light of thallium falls between the 

 Fraunhofer's lines D and E and very near to E. 



The only other data known to me for these numbers are, first, 

 one by Fizeauf for red lithium light derived from phenomena 

 of interference of another kind, A = 0*0006703 ; secondly, mea- 

 surements by M uller J in Freiburg, made according to the method 

 I employed. These gave 



A Li =0-0006763, A D = 0-0005918, A Th = 0-0005348. 



The instrument used by Miiller, a Babinet's goniometer, only 

 allowed readings to one minute. As Fraunhofer's numbers, 

 although not trustworthy in the fourth decimal place, are never- 

 theless the most exact, I attach the greatest value to my deter- 

 minations of the undular length of the lithium- and thallium- 

 lines which rest upon these data. The most probable errors, w } 

 of the final result of my observations are 



*= +0-6745 A /^±S±M±Hl 3 



V s(s-l) 



w Li =l"-5, w Na =l"-6, w Th = l"-9, 

 from which the accuracy of my results may be estimated. 

 [To be continued.] 



* Denkschrift der Miinchener Akademie aus dem Jahre 1823. Gil- 

 bert's Annalen, vol. Ixxviii. 



t hoc. cit. Ann. de Chim. et de PJiys. S. 3. vol. Ixvi. p. 429. I may 

 be allowed here to draw attention to a trifling error of Fizeau. This 

 physicist considers the yellow line of lithium /3 Li to be identical with the 

 sodium-line. That this is not the case is shown by Kirchhoff's data as well 

 as by simple inspection. 



X Pogg. Ann. vol. cxxviii. pp. 612-644. 



