31 



Formula (2) or the equivalent formula (5) expresses Babinet's 

 Rule which is here shown to be applicable, in the case of a mono- 

 electronic substance, only to one part of the spectrum, namely to 

 wave-lengths i > Ag. 



2nd. Suppose A <, Z(^ or n > Hq. 

 The discussion of this case may be conducted on exactly the same 

 lines as before. It will be seen that, in this case, 



I'l > Vo or c^ < c.^ ) ,. [ ^1 < J^a I fa\ 



j according as . (6) 



Vi < V2 or Ci > (-2 J ' I J^i > J<2 I 



The revolution in the ellipse is 



right-handed I ,. ( t/* > I ._, 



according as . (7) 



left-handed | | ^ < j 



This is the reverse of Babinet's Rule. Our equation (1) asserts 

 that it holds true, in the case of a mono-electronic substance, for 

 ixW values of Z inferior to Zq. 



§ 5. Circular dichroism in naturall}' - active media has been 

 discovered by Mr A. Cotton; he has investigated the rotation of 

 the major axis of the ellipse and the ratio of the semi-axes for a 

 number of solutions; and to his admiriible work^) we must turn 

 when we desire to ascertain in how far theoretical results are cor- 

 roborated by experimental evidence. Before doing so, however, some 

 preliminary remarks may be necessarj^. It must be borne in mind 

 that Mr Cotton attaches the positive sign to dextrogyric, the ne- 

 gative sign to laevogvric rotation; thus 



(Mr Cotton's) q = — (our) ip. ' 



The next remark called for is that Mr Cotton indicates the dir- 

 ection of the revolution in the ellipse by assigning to his angle cp 

 the positive or the negative sign . the positive corresponding to 

 right-handed, the negative to left-handed revolution. Borrowing fron^ 

 the numerical tables and from the graphical representations in Mr 

 C o 1 1 o n's These we have to remember, however, that, as has been 



"5 



1) Kecherches sur I'absorption et la dispersion de la Inmiere par les milieux 

 iiou^s du pouvoir rotatoire. These, Paris 1896; and Annates de Chimie et de Phy- 

 siqm VII Serie, tome VIII, p. 347, Paris 1896. 



