388 Royal Irish Academy. 



ducting experiments with the rhomh. The two cases are precisely- 

 similar ; and if it he 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, con- 

 sidering 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, respecting 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 any one 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 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 rhom- 

 boid of Iceland-spar, give two images always sensibly equal in in- 

 tensity. 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 observations, taking (suppose) the values of 6' 

 and ft' for the true values of 6 and ft ; or if, in the second method, 

 we contented ourselves with merely measuring the angles y' and y". 

 The necessity of attending to the foregoing rules and remarks will 

 appear from an examination of the experiments of M. de Senarmont, 

 published in the Annates de Chimie, torn, lxxiii. pp. 351-358. In 

 these very elaborate experiments, which were made upon light re- 

 flected at various incidences from steel and speculum metal, the 

 author followed a plan similar to that which I have adopted, and 

 which, in a general way, I had previously sketched in the Proceed- 

 ings of the Academy (vol. i. p. 159). There was this difference, 

 however, that he used a plate of mica instead of Fresnel's rhomb. 

 Now as he worked with common white light, the use of the mica 

 plate must have rendered two kinds of errors unavoidable. In the 

 first place, it would be impossible always to take the observations for 

 the same ray of the spectrum ; and next, as a consequence of this, 

 the thickness of the plate would be generally inexact for the parti- 

 cular ray to which the observations happened to correspond. If the 

 thickness of the plate were exact for a certain ray, it would be very 

 sensibly inexact even for the neighbouring parts of the spectrum ; 

 and as the part of the spectrum to which the observations belonged 

 was continually changing, the results obtained for different incidences 

 and azimuths would not be comparable with each other, even though, 

 in each separate case, the error of the plate were allowed for and 

 eliminated. The values of 6, however, as determined by M. de Se- 

 narmont, would be correct, so far as this error is concerned ; those 

 of ft alone would be erroneous. For the values of d were determined 



