BY REFLEXION FROM METALLIC SURFACES. 271 



4. In these cases the metallic reflexion performs the part of the first rhomb in 

 Fresnel's experiment with two. If I rightly interpret Sir D. Brewster's process 

 for a similar restoration, before referred to, and if the two metal plates are analogous 

 to the two rhombs, in the change of plane , which he so accurately determined, after 

 the second reflexion, at the maximum incidence, we may infer a correspondence with 

 that produced in the rhomb ; and that it is equal to twice the azimuth of the rhomb. 

 And in fact I find the results from two such different methods agree very closely for 

 all the pure metals ; though for certain ores, very low in the scale, there are some 

 discrepancies. 



There is also a close agreement between these results and the azimuths of the 

 plane of the ray restored to plane polarization by the action of a crystallized plate, 

 as given by the same author for some of the principal metals*, which confirms the 

 same inference. 



5. The changes of plane for successive incidences are in general of a nature analo- 

 gous to tliose in the reflexion from transparent bodies, but not the same. 



At the incidence for the maximum, if the plane of the polarizer be first adjusted 

 to give, e.g. the dark system, then on changing its plane to 45°, the analyzer, 

 in order to restore what is analogous to the same, that is, the darkest system, must 

 coincide with the plane of incidence, or its azimuth is 0°, in exact agreement with 

 what obtains both by theory and observation in the reflexion from transparent bodies. 



At greater incidences, to restore the same system, the analyzer must be moved 

 through increasing arcs on rnie side of the plane of incidence, and at lesser incidences 

 on the other, as for transparent bodies. 



Following up these latter, as the incidence is successively diminished, the azimuth 

 of the analyzer changes in a manner obviously different from that which obtains 

 for transparent bodies, though of the same general character : and the increase is 

 slightly different for different metals ; but in all cases it approaches 45° as the inci- 

 dence approaches the perpendicular. 



In these cases the azimuth of the polarizer remains at 45°. If it be changed, that 

 of the analyzer preserves a constant relation. 



Though these observations are of an obvious nature, yet, as far as I know, none 

 of the kind have been published for any series of metals. I have accordingly given 

 such a set for various reflecting substances. 



But though apparently simple in principle, the process is troublesome in practice, 

 and affected by various causes of uncertainty and difficulty. Some apparent anoma- 

 lies, especially, which caused the arcs at small incidences to appear to increase 

 beyond 45°, for a long time caused me much perplexity. 



The results however here offered, having been obtained with the use of every pre- 

 caution, and being the means of a great number of repetitions, I trust, on the whole, 

 may not be useless as a first attempt to determine these changes by direct observation. 



6. The ellipticity at different incidences cannot be deduced (except at the max- 



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



