304 Lord Kelvin '-on the Application ofSellmeier's Dynamical 

 Hence if we put 



T =i(« + « y )(l+ i ^ 5 ) ..... (3); 



and if x be any numeric not exceeding 4 or 5 or 10, we 

 have 



V 2 -fc 2 ~ 2x+V T 2 -yc y 2_T 2¥-l * ' ' ^' 



Using this in (1), and denoting by //, the refractive index 

 from ether to an ideal sodium-vapour with only the two 

 disturbing atoms m, m n we find 



_ 1000 m 1000 m, ,.. 



2# + l 2#— 1 v J 



whence 



f 2 . 1000 t 2 . 1000 



©'-- 



§ 4. "When the period, and the corresponding value of x 

 according to (3), is such as to make fi 2 negative, the light 

 cannot enter the sodium-vapour. When the period is such as 

 to make /jl 2 real, the proportion (according to Fresnel, and 

 according to the most probable dynamics,) of normally incident 

 light which enters the vapour is 



*- W (7). 





§ 5. Judging from the approximate equality in intensity 

 of the bright lines D l3 D 2 of incandescent sodium-vapour ; 

 and from the approximately equal strengths of the very fine 

 dark lines D b D 2 of the solar spectrum ; and from the ap- 

 proximately equal strengths, or equal breadths, of the dark 

 lines !>!, D 2 observed in the analysis of the light of an incan- 

 descent metal, or of the electric arc, seen through sodium- 

 vapour of insufficient density to give much broadening of 

 either line ; we see that m and m, cannot be very different, 

 and we have as yet no experimental knowledge to show that 

 either is greater than the other. I have therefore assumed 

 them equal in the calculations and numerical illustrations 

 described below. 



§ 6. At the beginning of the present year I had the great 

 pleasure to receive from Professor Henri Becquerel, enclosed 

 with a letter of date Dec. 31, 1898, two photographs of ano- 



