332 
appear not wholly unwarranted if we continually bear in mind that 
they are at best only approximations, to be given up as soon as 
further progress will be made. 
Let 
( 1 ) 3neï ~e^ = A 
Lorentz’ equation [(6) and (7), § 6.] may then be written in the 
case of a mono-electronic substance: 
( 2 ) 
In 
^äl- A ( 1 L) 
V 2 — I Vi 0 2 It)- 
many cases v — 1 is small so that (2) becomes approximately 
(3) 
where 
3 
2{v 1) 
(4) D = A/X 0 * 
If À 2 are two wave-lengths and v 2 the corresponding indices 
, 5) A = 
In what follows, all wave-lengths are expressed in terms of the unit 
(6) 10~ 5 cm. 
throughout; all values of the constant A are expressed in terms of 
the unit 
(7) 
10 7 cm 2 
throughout; these factors will be omitted. The values of D are pure 
numbers. 
All values of v and of K given below are understood to refer 
to gases considered at 0° C. and 760 millims. 
§ 16. Hydrog en. Equation (3) of the preceding section has 
been applied by H. A. Lorentz to Ketteler’s measurements of the 
indices of hydrogen for Fraunhofer’s lines 6r, Z>, B. The values of 
the constants found are as follows 1 ): 
(1) D = 10707 A = 7,39. 
9 K on. A k ad. v. Wet. te Amsterdam, Verslagen d. W. e. Nat. Afd., 
Deel VI (1898) p. 513. 
