1871.] 



Successive Polarization of Light. 



383 



extraordinary index. But if we take a lamina of a negative crystal, in 

 which the extraordinary index is the least, as a film of Iceland spar split 

 parallel to one of its natural cleavages, the phenomena are the reverse of 

 those exhibited by quartz : when the principal section is on the left of the 

 plane of reflection the colours descend, and when it is on the right of the 

 same plane the colours ascend, the analyzer being turned from left to 

 right. 



It has been determined that the ordinary ray, both in positive and 

 negative crystals, is polarized in the principal section, while the extra- 

 ordinary ray is polarized in the section perpendicular thereto. It is also 

 established that the index of refraction is inversely as the velocity of 

 transmission. It follows from the above experimental results, therefore, 

 that when the resolved ray whose plane of polarization is to the left of the 

 plane of reflection is the quickest the successive polarization is right- 

 handed, and when it is the slowest the successive polarization is left- 

 handed — in the order R, O, Y, G, B, P, V, and in the second case in the 

 reversed order. 



The rule thus determined is equally applicable to laminae of biaxal 

 crystals. 



As selenite (sulphate of lime) is an easily procurable crystal, and readily 

 cleavable into thin laminae capable of showing the colours of polarized 

 light, it is most frequently employed in experiments on chromatic polar- 

 ization. The laminae into which this substance most readily splits, 

 contain in their planes the two optic axes ; polarized light transmitted 

 through such laminae is resolved in two rectangular directions, which 

 respectively bisect the angles formed by the two optic axes : the line which 

 bisects the smallest angle is called the intermediate section ; and the line 

 perpendicular thereto which bisects the supplementary angle is called the 

 supplementary section. These definitions being premised, if a film of sele- 

 nite is placed on the diaphragm with its intermediate section to the left of 

 the plane of reflection, the successive polarization is direct or right-handed ; 

 if, on the contrary, it is placed to the right of that plane, the successive 

 polarization is left-handed. The ray polarized in the intermediate section 

 is therefore the most retarded ; and as that section is considered to be 

 equivalent to a single optic axis, the crystal is positive. 



In one kind of mica the optic axes are in a plane perpendicular to the 

 laminae. They are inclined 22\° on each side the perpendicular within 

 the crystal, but, owing to the refraction, are seen respectively at an angle 

 of 35°'3 therefrom. The principal section is that which contains the two 

 optic axes. If the film is placed on the diaphragm with its principal 

 section inclined 45° to the left of the plane of reflection, the successive 

 polarization is right-handed. The ray, therefore, polarized in the section 

 which contains the optic axes is the one transmitted with the greatest 

 velocity. 



Films of uniaxal crystals, whether positive or negative, and of biaxal 



2 g 2 



