ee 
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OF CALCAREOUS SPAR. 167 
closes it ; and from this circumstance arise all the phenomena 
we have mentioned. When a ray enters the surface BFda, or 
BF cd, it is divided into two pencils, polarised in opposite planes ; 
and when these two pencils reach the interrupting stratum 
abcd, 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 ADdcdE; 
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 asecond prism, can vanish in any part 
of its revolution.. Henee, 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 abcd, have their opposite polar- 
ised portions arranged into two pencils, polarised in an oppo- 
site manner, by the action of the prism BadcFd. 
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 abcd has the power of completely denolarising 
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 iaclimeth 45° 
to the axes of the other; and hence it will follow, that the film 
abcd has the same properties as if it had been detached from 
the place a b’ba on the upper surface ABCD, and had slid 
down into the position abcd. ‘This inference is fortunately 
capable of the most rigid demonstration. Let MBEN, Fig. 8. 
be the principal section of the rhomboid ; or a section of Fig. 7. 
on the line FBf, and let fgqp bea ‘neteaaheg| section of ihe 
interr upting fag 
