M. MELLONI ON THE POLARIZATION OF HEAT. 159 



stant in all the series, and is not much less than complete polar- 

 ization, or |§§. It would, without doubt, arrive at it if the 

 optical axes of the different mica laminae which compose each 

 of the two piles were exactly in the direction required for render- 

 ing the proper action of the crystallization totally inappreciable, 

 and if the rays introduced into the system were all exactly par- 

 allel, conditions which it is extremely difficult, not to say im- 

 possible, rigorously to fulfil. By substituting my eye in the 

 place of the thermoscopic body, when the two planes of refrac- 

 tion intersected, I constantly perceived, through the system, 

 traces more or less decided of coloration. These colours, due 

 to the action of the central laminae, showed definitively that the 

 light itself did not undergo a complete polarization under the 

 influence of the mica piles : and 1 have little doubt that, had it 

 been possible to measure with precision their degree of lumi- 

 nous polarization in the oblique positions, at which they gave 

 their maximum effect, the value would have been found to be 

 nearly -^^^f as for the greatest effect of calorific polarization. 



M. Biot had previously remarked that the proportion of light 

 polarized by refraction, is increased indefinitely with the angle 

 of incidence, so that the maximum effect occurs at the greatest 

 degree of obliquity at which the rays of fight can penetrate the 

 substance of which the refracting laminae are formed. 



Sir D. Brewster has, besides, enunciated that the light of a 

 wax candle, at a distance of from ten to twelve feet, is completely 

 polarized, at an inclination of 10° 49', by eight plates of crown 

 glass; at 32° 50', by twenty-seven plates; and at 48° 19', by 

 forty-seven plates ; so that setting out from perpendicular inci- 

 dence, the angle limit, at which complete polarization com- 

 mences, is so much nearer the normal in proportion to the am- 

 plitude of the number of the polarizing laminae. 



The laws, therefore, of polarization by refraction, in reference 

 both to heat and light, are exactly similar. 



A very simple observation upon the numbers contained in the 

 second or third column of the last six tables, will show that the 

 calorific rays are also polarized by reflection ; that in this case 

 there is a given incidence at which the polarization takes place 

 in the highest degree ; and that the planes of the two polariza- 

 tions, produced upon the radiant heat by the action of the 

 forces of refraction and reflection, are respectively perpendicular. 



If we look at an object through a lamina of glass, or any other 



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