462 THEORY OF OPTICS
lected, i.e. provided the substance has no appreciab tion,
« ~ //•' — i.
I ^./"/r'-Yr"
In this equation // is the index of refaction A> l/ie period Tf .-- 2?rrf when the substance is at res tion (12) is derived from the theory of dispersion |ef" (18) on page 387]. If now in equation (n) the (
coefficient ,, be replaced by its value in terms of ' • t
(tl t, 7
(3), and if the resulting value for 4>r/v be substitute* differential equation is obtained for the substance referred to a fixed system of coordinates. This e much simplified if only terms in the first order in *' In It is always permissible to neglect tin* other terms, s when 7* represents the velocity of the earth in space very small in comparison to the velocity of light.
possible to replace . by ,'.' in those terms in (n)
tit t !/
multiplied by 7% and also to neglect, in the case geneous substances, the second term of ( n j which is by ?'^, since approximately, i.e. for «*' - o, for change of condition in such substances the followin
holds (cf. page 275):
Thus (n) becomesh wil later, in which v: c has a still smaller value, it is only the terms <evyy evs. The magneticn this point. This matter will be more fully discussed below.