( 384) 



roi' -- > « > O, — ^A'^0 tlie cxtiiic'tioiis successivelv beL'ume( — ^'-f-.'/') 

 and ( — ^" + //') ; we find llie followinu- values: 



(f' z=z y' — 6\ 





When (i = 90° and ,/; = | - — V |=:46 30' the cxtiiu-tion Iiccunies 



indefinite; it is here that tlie transition from 110^ to 0" takes i)kicc, 

 In the same way we find for 



Also here the extinction for ^f — 90° and ,i = U^ — r =4fi\30' 



becomes indefinite. 



The shape of the r/:-isogyres is I'epresented in tig. 6. The black 



lines refer to <f>\ those in black and white to ip ; y" gives the value 



of the [»ositi\e exiiuclion with regard to ihe trace of //' (100), ^' 



