Circular Polarization. 559 



cases, the intricacy of the subject, and some deviations from 

 the Jaws of light, as established in Sir David Brewster's re- 

 markable paper on this subject*, demand a more prolonged 

 investigation than I have yet been able to give to it. In the 

 hope of being able to resume it in another paper, I content 

 myself at present with a reference to the facts respecting Me- 

 tallic Reflection, communicated to the Royal Society of Edin- 

 burgh on the 21st of March 1836, and printed in their Pro- 

 ceedingsf. 



* Philosophical Transactions, 1830. 



t The following is the Memorandum on the subject, extracted from the 

 Proceedings of the above date. 



" I have recently ascertained the following facts respecting heat. 



" 1st, That heat polarized in any plane, and then reflected from the sur- 

 face of a refracting medium, changes its plane of polarization in a manner 

 similar to what obtains in the case of light. Thus,, with a thick plate of 

 mica, which polarized homogeneous red light most completely at an inci- 

 dence of about 59°, the plane of polarization of reflected polarized heat 

 remained on the same side of the plane of reflection when the incidence 

 was greatf and was on the contrary side when the incidence was small. 

 The limiting angle of incidence was about 57°, which therefore should be 

 the polarizing angle of dark heat for mica. This mode of observing the po- 

 larizing angle offers some advantages above more direct methods. 



" 2ndly, Metals polarize heat extremely feebly by reflection. I have 

 carried my experiments up to 85° from silver, yet even there but a small 

 share is polarized. The effect is, however, distinctly recognisable through 

 a considerable range of incidences. The effects are such as would indicate 

 the maximum polarizing angle to be even much higher ; perhaps it never 

 attains a maximum. This fact corresponds to that in the case of light, ex- 

 cept that the maximum polarizing angle is 73° (Brewster). Did metals act 

 on light like other bodies, we should conclude, from the polarizing angle 

 being greater, that heat is more refrangible than light. An important re- 

 mark of Sir D. Brewster's, however, shows the statement I have made to 

 be in conformity with the views of the nature of heat which I have pub- 

 lished. He finds the maximum polarizing angle to be greatest for the least 

 refrangible rays. 



" 3rdly, Heatpolarized in a plane inclined 45° to the plane of subsequent 

 reflection at silver, has its nature changed as in the case of light, and pre- 

 sents the conditions of elliptic polarization, though the ellipse is much 

 more elongated, even at great angles of incidence. 



" 4thly, T?i;o reflections from silver increase the polarizing effect of metals. 

 This fact has its counterpart in light. Two reflections likewise produce an 

 increased tendency to circular polarization when the plane of reflection is 

 inclined 45° to that of primitive polarization. This effect increases with 

 the obliquity of incidence up to considerable angles. 



" These observations have been verified in the case of heat from various 

 sources, obscure as well as luminous." 



