( <in<t ) 



ha\'o heen ralliei- lai'ii'er lliaii iiiiitv. In iny npinioii, we uiiist go 

 farthei' than that anti ascrihc in j^ a \ahii'. nol seusihlv aho\e 1, 

 niv artiiiniejil lieiiig ihal llic (h»\ nnneli\ can niilv make itself felt, 

 if the (ntforeiice between the (hstanees from the original line lo the 

 two conipfnients in qnestion is not ven nineh smaller than the breadth 

 of the line. 



We loiow already (§ {() that I = at the middle of the line and 

 5 ^ r»; at the bordei-. Now, if j, were sensibly larger Ihaii i, the 

 places corresponding to ^^<> and 5:=!, i.e. the places occnpied 

 by the two c(niiponents in a weak field, wouhl lie within the 

 breadth (if llic original line : il wdnld llierefoi-e be impossible to 

 discern the want of symnielry. 



§ 18. Whatever be the exact \aliie of ij, Zeem.vn's experiments 

 on this point show at all e\ents ihal under faxonraltle circumstances 

 a displacenieni «if a Hue, corresj)Ondiiig to a change from 5 = to 

 § = 1, or to a change 



^ (38) 



of the freipiency, is large enough lo be seen. Rnt, if such is the 

 case, we shall no longer be right, if we discuss the \alue of ^, in 

 omitting (piaiililies dial arc^ but a few times smaller than uiuty. 



A (piantity (if this kind is ihc icrm a in the equation (11), which 

 as has already been meulioued. is but little tlillerent from '/,, and 

 wliicli we have onulled 111 all our calcidations. If we wish to lake 

 it into account, we ■-hall liiid llial all llial precedes will slill hold, 

 pro\id('(l (inly we replace //„ by die ipianlily //„. tlelcrmined by 



/" — n = >n' ,i\: • (39) 



Indeed, (2!S) may then be written in the Ibrni 

 i — m' (//.,' — ,r) , 

 and llic |ilace (if maxiniuin absorption, llic middle (if llu^ line, will 

 correspond to the fretpiency /a„, exactly as il tbrmerly correspdiuled 

 to the frequency //'„. 



Now, l)y (7j and (lt> 



/ = ;w' /»„', 

 and by (39) 



« « 



«7 — )(/ , , h'„ = «„ , .... (40) 



or, on account of (10), 



a y e' 

 ".. = "0-^ (41) 



We learn from this equation that an increase of the density must 



