388 



similar ; and if it be necessary not to neglect the errors of 

 the rhomb, it is certainly not less necessary to take into ac- 

 count those which may arise from a want of accuracy in the 

 thickness of the plate, considering 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, re- 

 specting the mode of making experiments on elliptic polari- 

 zation, because the subject is one which has not hitherto 

 been studied ; nor does it seem to have occurred to any one 

 that any precaution was requisite beyond that of getting the 

 rhomb cut as nearlv as possible at the proper angle, or the 

 crystalline plate made as nearly as possible of the proper 

 thickness. This, indeed, was quite sufficient 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 polarized 

 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 analysed by a rhomboid of 

 Iceland-spar, give two images always sensibly equal in inten- 

 sity. But an error which could not be at all detected in this 

 way, might produce a very great effect in such experiments 

 as those upon the metals, and, for the purpose of comparison 

 with theory, might render them entirely useless, if in the 

 first method of observing we relied upon one set of obser- 

 vations, taking (suppose) the values of 6' and j3' for the true 

 values of 9 and /3 ; or if, in the second method, we contented 

 ourselves with merely measuring the angles 7' and y". 



The necessity of attending to the foregoing rules and re- 

 marks will appear from an examination of the experiments 

 of M. de Senarmont, published in the Annates de CJdmie, 

 torn. Ixxiii. pp. 351-358. In these very elaborate experi- 

 ments, which were made upon light reflected at various inci- 

 dences from steel and speculum metal, the author followed 



