BY REFLEXION FROM METALLIC SURFACES. 279 



vation at each incidence. For some of the ores, &c. which reflect but little light, 

 the arcs must be taken as mere estimations. I have found some peculiar discord- 

 ances between different sets of observations, especially in silver and copper : those 

 given are the means. 



The observations here annexed in a tabular form consist of, — 

 1. Those with Fresnel's rhomb for ascertaining from the azimuth (y) for the re- 

 storation of plane polarization, the ellipticity at the incidence for the maximum ; or 



These results for the principal metals and other substances are seen in Table I., 

 columns 3 and 4. 



Again, in the observations with the rhomb at azimuth y, the plane of the emergent 

 ray is inclined by 2 y to the original plane. In Sir D. Brewster's experiments with 

 two metal plates it is inclined 45+'p. The results are here compared in columns 

 1 and 2, and sufficiently show that we have 



45 + <p 



for all the pure metals. 



Also we may remark, that Sir D. Brewster's empirical ellipse has the ratio of its 

 axes determined by sin 2 (p, and is therefore different from the undulatory. 



The same author also gives* the azimuths of the plane of the ray restored to plane 

 polarization by a crystallized plate for several metals, as follows : — 



Pure silver . . . 42° Speculum ... 32*^ 



Copper .... 36° 30' 

 Mercury .... 35° 

 Platinum .... 34° 



)0 



Steel ..... 30° 30' 



Lead 26° 



Galena . . . . 17° 30' 



In each instance it will be seen (on comparison with Table I.), that as nearly as 



45 -Lm 



possible the arc = — ~ ; with the exception of galena. 



2. Observations for the values of x at successive incidences, while | remains at 45°. 

 These series of arcs are given in Table II., column 1, for the two metals of greatest 



and least maximum ellipticity and two intermediate ; and in Table III. for a number 

 of others. 



3. Observations for the ellipticity at different incidences. 



In general the ellipticity is measured by the inclination of the rhomb (y) to the 

 plane of previous polarization. 



For the maximum ellipticity, since %=0, it follows that y is correctly measured 

 from the plane of reflexion. But at other incidences y must be corrected by the 

 change of plane of polarization due to that incidence, or we must take (y — %). 



In Table II. (columns 2 and 3) are given the observed values of y for four principal 



» Philosophical Transactions, 1830, Part II. p. 311. 



