406 



of iron increases at higher temperatures in a greater ratio than that 

 of platina. The discrepancy between the temperatures derived from 

 the observations with his first pyrometer and the present one, he ad- 

 mits to be considerable, but believes they may be sufficiently ac- 

 counted for by the differences in the circumstances of the experi- 

 ments, without imputing inaccuracy to the instrument. The author 

 next attempted to ascertain the effects of the most intense heat which 

 it was possible to produce in a furnace, and to measure the utmost 

 limits of expansion in a platina bar ; but various circumstances in- 

 terfered with the success of these experiments, which afforded, how- 

 ever, many curious results as to changes of integration in platina by 

 the effects of heat. The paper concludes with some observations on 

 the practical advantages possessed by the present instrument. 



On the Phenomena and Laws of Elliptic Polarization, as exhibited in 

 the Action of Metals upon Light. By David Brewster, LL.D. F.R.S. 

 L. * E. Read April 22, 1830. [Phil. Trans. 1830, p. 287.] 



The action of metals upon light has always presented a remarkable, 

 and hitherto inexplicable, anomaly in the science of polarization. 

 Malus, to whom this branch of optics owes its origin, had at first an- 

 nounced that metals exerted no polarizing influence on light ; but 

 Dr. Brewster, by employing a different method of observation, ascer- 

 tained that the light reflected from metallic surfaces was modified in 

 such a manner as to exhibit, when transmitted through thin crystal- 

 lized plates, the complementary colours of polarized light. He af- 

 terwards discovered the curious property possessed by silver and gold, 

 of dividing a polarized ray into complementary colours by successive 

 reflexions. M. Biot, to whom the author communicated this disco- 

 very, pursued the inquiry to which it led, and arrived at the same 

 conclusions as to the mode in which this class of phenomena should 

 be explained. Subsequent researches, however, convinced the author 

 that these generalizations had been too hastily formed ; and the study 

 of Fresnel's curious discoveries' respecting circular polarization ena- 

 bled him to advance still further in the inquiry, and he now presents 

 to the Royal Society in this paper, a complete analysis of the singular 

 phenomena exhibited in the action of metals upon light. 



The first section of the paper treats of the action of metals upon 

 common light. A ray of common light reflected from a metallic 

 surface when analysed by a rhomb of calcareous spar, exhibits a de- 

 falcation of light in one of the images, as if a portion of the light 

 was polarized in the plane of reflexion. This effect will be still more 

 distinctly seen on examining the system of polarized rings formed 

 round the axes of crystals by means of the light reflected from me- 

 tals. If the light had s JFered no modification by reflexion, or if the 

 metal reflected in equal quantities the light polarized in opposite 

 planes, the rings would not be visible at all, whereas it is found that 

 they are easily visible in the light reflected from all metals. They 

 are most distinctly perceived at an incidence of about 74, and be- 



