387 



rhomb were erroneous, the true value of 9 would still be 

 half the sum of 7', 7"; but the true value of j3 would not be 

 discoverable without measuring the angles 6', 9", by the help 

 of which it can be deduced from the second of formulae (n), 

 combined with the second of formulag (k). Nor can we dis- 

 cover whether the rhomb is erroneous or not, without mea- 

 suring the angles 9', 9"; and therefore as these angles must 

 be measured in any case, the former method of determining 

 9 and /3 is to be preferred. 



In making experiments on elliptically polarized light, a 

 plate of mica or any other doubly refracting crystal, placed 

 perpendicular to the ray, may be used instead of Fresnel's 

 rhomb. If the thickness of the crystalline plate be such that 

 the interval between the two rays which emerge from it is 

 equal to the fourth part of the length of a wave, for light 

 of a given refrangibility, the plate will, for such light, per- 

 form all the functions of the rhomb; the principal plane of 

 the rhomb being represented by the plane of polarization of 

 one of the emergent rays. But unless the light be perfectly 

 homogeneous, this method is liable to great inaccuracy in 

 practice, since the effect of the plate in producing or altering 

 the difi^erence of phase between the two rays which interfere 

 on their emergence from it, is inversely proportional to the 

 length of a wave, and will therefore be extremely different 

 for light of different colours, and will change very per- 

 ceptibly even within the limits of the same colour. It is true, 

 the effect of the rhomb also varies with the colour of the 

 light: but this variation is trifling compared with that which 

 exists in the other case. It was for this reason that I em- 

 ployed the rhomb in my experiments, instead of a crystalline 

 plate. The apparatus, however, is much simplified by using 

 such a plate ; and if any one chooses to do so, and to work 

 with homogeneous light, he must take care to follow, in every 

 respect, the directions which I have given for conducting 

 experiments with the rhomb. The two cases are precisely 



