406 PROFESSOR STOKES, ON THE COMPOSITION AND RESOLUTION OF 



the case, the two pencils, polarized otherwise than oppositely, into which a polarized pencil 

 is resolved on entering into the medium, would at emergence compound a pencil of which the 

 intensity would depend upon the retardations of phase of one pencil relatively to the other, 

 so that such a medium, when examined with polarized light, ought to exhibit rings or 

 colours without the employment of an analyzer. It is here supposed that the light enters 

 the medium at the first surface, and leaves it at the second, in a direction normal to the 

 surface, or very nearly so, and likewise that the refracting power of the medium for the two 

 pencils is not very different, so that the effect of reflexion at the two surfaces may be disregarded, 

 the only sensible effect being to diminish the intensity of each of the two pencils in the same 

 proportion, without affecting its state of polarization. These views would lead us to scrutinize 

 very carefully any experimental evidence brought forward which would lead to the conclusion, 

 that the two polarized pencils which a doubly refracting medium was capable of propagating 

 in a given direction were polarized otherwise than oppositely. 



7. In ordinary doubly refracting crystals, whether uniaxal or biaxal, and in doubly 

 refracting liquids, such as syrop of sugar, it is generally admitted that the two pencils 

 transmitted in a given direction are oppositely polarized. In the former case the two pencils 

 are polarized in rectangular planes, in the latter they are circularly polarized, one being 

 right-handed and the other left-handed. The same is the case with quartz for pencils 

 transmitted in the direction of the axis ; but in following out the researches in which he so 

 successfully connected the phenomena, identical with those of a doubly refracting liquid, 

 which quartz exhibits in the direction of the axis, with the phenomena which it exhibits as 

 a uniaxal crystal, Mr Airy met with an experimental result which seemed to shew that 

 while the ellipses which characterize the two streams transmitted in a given direction, oblique 

 to the axis, have their major axes situated, one in a principal plane, and the other perpen- 

 dicular to the principal plane, and the directions of revolution are opposite, the eccentricities 

 of the ellipses, though nearly, are not quite equal*. This conclusion depended upon the 

 result of certain experiments made by means of a Fresnel's rhomb. The nature of the ex- 

 periments seemed to eliminate the effect of an error in the rhomb, that is, a deviation from 

 90° in the retardation of phase which it produced in a pencil polarized in the plane of re- 

 flexion relatively to a pencil polarized in a plane perpendicular to the former. The effect of 

 a possible index error in the plane of reflexion seemed also to be eliminated ; and the 

 quantities on which the results depended, though small, seemed to be beyond mere errors 

 of pointing. Being impressed however with a strong conviction that the result depended in 

 some way on the mode of observation, I was led to scrutinize the different steps of the 

 process, and it occurred to me that the apparent inequality of eccentricities was probably 

 due to defects of annealing in the rhomb. It is next to impossible to procure a piece of 

 glass of such a size free from defects of that nature, for which reason I believe that even a good 

 Fresnel's rhomb is not to be trusted for minute quantities, except in the case of merely 

 differential observations. 



* Cambridge Philosophical Transactions, Vol. IV. p. 204. 



