242 Laws of Metallic Reflexion, and Mode of 



of the rhomb ; the principal plane of the rhomb being repre- 

 sented 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 difference 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 ex- 

 tremely different for light of different colours, and will change 

 very perceptibly even within the limits of the same colour. It 

 is true, the effect of the rhomb always 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 employed 

 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 homo- 

 geneous light, he must take care to f oUow, in every respect, the 

 directions which I have given for conducting experiments with 

 the rhomb. The two cases are precisely similar; and if it be 

 necessary not to neglect the errors of the rhomb, it is certainly 

 not less necessary to take into account those which may arise 

 from a want of accuracy in the thickness of the plate, consider- 

 ing how difficult it is to make the thickness correspond exactly 

 to the particular ray which we wish to observe. 



I have been induced to enter into these particulars, respect- 

 ing the mode of making experiments on elliptic polarization, 

 because the subject is one which has not hitherto been studied; 

 nor does it seem to have occurred to anyone that any precaution 

 was requisite beyond that of getting the rhomb cut as nearly as 

 possible at the proper angle, or the crystalline plate made as nearly 

 as possible of the proper thickness. This, indeed, was quite suf- 

 ficient for ordinary purposes. For example, light polarized in a 

 plane inclined 45 to the principal plane of the rhomb or of the 

 plate, would, as far as the eye could judge, be circularly pola- 

 rized after passing through either of them. Notwithstanding a 

 certain error in the angle of the one, or in the thickness of the 

 other, such light would, when analyzed by a rhomboid of Iceland- 



