OF CALCAREOUS SPAR. 167 



closes it ; and from this circumstance arise all the phenomena 

 we have mentioned. When a ray enters the surface TIF da, or 

 BFc6, it is divided into two pencils, polarised in opposite planes ; 

 and when these two pencils reach the interrupting stratum 

 abed, they are depolarised by that stratum, in the same manner 

 as if they had been acted upon by a film of sulphate of lime, and 

 each pencil consists of two pencils, oppositely polarised, but not 

 separated from each other. These two compound pencils have 

 their polarisation again changed, by the flat plate ADZ>c«?E ; 

 but as it cannot separate the oppositely polarised portions, the 

 two pencils emerge from the face ADHE in such a confused 

 state, that none of the four pencils into which they are subdivi- 

 ded by the application of a second prism, can vanish in any part 

 of its revolution. Hence, the reason is obvious, why the pen- 

 cils vanish when the ray of light is first incident upon the face 

 ADHE ; for in this case, the compound pencils, when trans- 

 mitted through the stratum abed, have their opposite polar- 

 ised portions arranged into two pencils, polarised in an oppo- 

 site manner, by the action of the prism BabcFd. 



Let us now endeavour to determine what is the position of 

 the axes of the film relative to those of the surrounding mass. 

 Since the film abed has the power of completely depolarising 

 the incident light, we are entitled to infer, that its depolarising 

 axes are coincident with the neutral axes of the rhomboid ; or, 

 what is the same thing, that the axes of the one are inclined 45° 

 to the axes of the other ; and hence it will follow, that the film 

 abed has the same properties as if it had been detached from 

 the place a b' b a on the upper surface A BCD, and had slid 

 down into the position abed. This inference is fortunately 

 capable of the most rigid demonstration. Let MBFN, Fig. 8. 

 be the principal section of the rhomboid ; or a section of Fig. 7. 

 on the line FBf, and let fg qp be a magnified section of the 



interrupting 



