356 Mr. R. Moon on Polarisation and Double Refraction. 



reflecting or refracting surface. Hence if a series of parallel 

 plane waves be transmitted through a doubly refracting cry- 

 stal, there tan be none of that polarization in opposite planes 

 of the incident and emergent rays, of which so much has been 

 said. Although unable to support myself by any reference to 

 the results of actual experiment, I would with all humihty beg 

 leave to doubt whether this polarization of opposite planes of 

 the emergent rays, corresponding to the same incident pencil 

 of parallel rays, do really obtain. No fact in physical or plane 

 optics can be more incontestable than this, that whenever a 

 pencil of parallel rays is incident on the surface of a refracting 

 body, bounded by parallel planes, the emergent pencil, what- 

 ever be its course within the medium, must be parallel to the 

 incident rays. Yet in a work so universally received as the 

 treatise on Light of Sir John Herschel, we find it broadly 

 stated, that if a series of parallel waves be incident on a 

 doubly refracting crystal, bounded by parallel surfaces, the two 

 unequal rays will be inclined to one another. This viay be 

 the fact, though certainly if so it militates vitally against the 

 undulatory doctrine of reflexion, since upon that doctrine, as 

 I conceive, it is impossible to hold any but a contrary opinion ; 

 but it is possible that this apparent fact may be resolved as 

 follows : — Assuming that the emergent rays are always par- 

 allel to the incident, it would result that the two pencils of 

 rays, by which any small object is made visible through a 

 doubly refracting crystal, arise from two radically distinct 

 pencils of the incident light, the inclination of which to each 

 other would be exactly the same as that of the two emergent 

 pencils; and this may possibly be the reason why it came to 

 be believed that the emergent rays corresponding to a pencil 

 of parallel waves were inclined to each other. 



That this is the true state of the case, in the absence of ex- 

 periment it is impossible to decide, but assuming it to be so, 

 it is easy to see how the further mistake should have crept in, 

 of supposing the two emergent pencils arising from the same 

 incident pencil to be polarized in different planes. The emer- 

 gent pencils, by which a small object is made visible through 

 a doubly refracting crystal, are due to different pencils of the 

 incident light; the plane of polarization of each emergent 

 pencil will be perpendicular to the intersection of the waves 

 composing the corresponding pencil of incident light with the 

 refracting surface; therefore the two planes will necessarily be 

 inclined to each other, but they will not necessarily be at right 

 angles. 



