M. MELLONI ON THE POLARIZATION OF HEAT. 173 



It would be superfluous, for the end proposed, to enter into 

 all the theoretical details relative to the different modifications 

 that the lamina interposed impresses upon the luminous pencil 

 in proportion as its principal section turns around the plane of 

 primitive polarization ; they may be found in all optical trea- 

 tises. Let it suffice that we call to mind that the equality of the 

 two variations of which we have recently treated, is a necessaiy 

 consequence of the double refraction and the complete and rect- 

 angular polai-ization that the luminous pencil undergoes in the 

 interior of the bi-refracting crystal. The light polarized by the 

 first mu-ror, when traversing this thin crystal, is either sub- 

 divided into two parts sensibly superposed, or presences its 

 unity, according as any one of the neutral axes is inclined, or 

 parallel to the plane of primitive polarization. When the sub- 

 division takes place, there results from it, at the particular incli- 

 nation of 45°, two pencils of equal intensity, ordinaiy and ex- 

 traordinary, which, in the two cases under examination, always 

 have their planes of polarization so turned that one of them is 

 found precisely comprised in the plane of reflection of the second 

 miiTor, and the other in the perpendicular direction : it is the 

 first only which can undergo the second reflection and reach the 

 eye. Now one of these two pencils is sometimes added to the 

 light reflected by the second mirror, and sometimes subtracted, 

 -which is the reason that the augmentation produced, Mhen the 

 planes of reflection are perpendicidar, is equal to the diminution 

 which takes place when these planes are parallel. 



The results just related do not absolutely require the employ- 

 ment of two mirrors, but may be also obtained with a pair of tour- 

 mahnes, whose axes are rendered successively parallel or perpen- 

 dicular. They may also be obsen^ed by means of two series of 

 parallel laminae of glass, properly mclined to the incident rays, 

 and so disposed as that their planes of refraction are sometimes 

 parallel, at others perpendicular. 



Now, if the same phcenomena can be produced upon calorific 

 rays, we may conclude that heat is refracted and polarized, like 

 light, in bodies possessed of double refraction. This experiment 

 was tried by Mr. Forbes with two of his mica piles, giving 

 from twenty to thirty hundredths of sensible polarization, be- 

 tween which he interposed a large vertical lamina of mica, 

 which was provided with two contiguous bases, and inclined to 

 each at an angle of 135°. The principal section having a direc- 



VOL. II. PART VI. M 



