M. MELLONI ON THE POLARIZATION OF HEAT. 179 



be equal to that observed upon the preceding source. We then 

 pass to the augmentations produced in both cases, by inclining 

 the principal section 45° upon the plane of refraction of the an- 

 terior pile ; these two augmentations are again respectively equal. 

 In aU these experiments the indicating needle of the galvano- 

 meter moves over a considerable space, for we have recently seen 

 that it sometimes describes arcs which exceed 30°. The smaller 

 angle described by virtue of the alteration of the direction of the 

 principal section is due to the action of the heat thrown off by 

 copper at 400°, which scarcely propels the needle beyond 7° ; 

 but as, by means of the artifice just indicated, the heat of flame 

 may be employed to a sufficient extent to give precisely the same 

 movement, the equahty of the depolarization in these two extreme 

 cases is proved in the most evident manner. 



The two pencils of fight produced by the plate of mica or sul- 

 phate of lime, in positions in which the principal section is in- 

 cfined 45° upon the primitive plane of polarization interfere when 

 they are reflected together by the second mirror, or transmitted 

 by the second pile, and thus develope the beautiful colours treated 

 of above. Is there an analogous interference of the calorific rays ? 

 Coloration being here the criterion of the interference, I at 

 first thought that I should easily succeed in verifying the exist- 

 ence of this phenomena in heat by experiments of diathermancy. 

 I will endeavour to explain my ideas more clearly. 



We know that the two coloured images obtained by the inter- 

 position of the plate of mica or sulphate of fime, having the prin- 

 cipal section incHned45° upon the plane of primitive polarization, 

 whilst the second plane of polarization is rendered alternatively 

 parallel or perpendicular to the first, have always complementary 

 tints. We will suppose that these tints are the red and the 

 green. If we A^iew these two images produced thus successively 

 through a glass of a very pure red colour, the first will be seen 

 and not the second. If, instead of red, white or some other kind 

 of coloiu'ed glass be employed, the two images will be seen, 

 sometimes in their natural state, and sometimes altered; the 

 red image more than the green, or the green more than the red, 

 according to the nature of the screen glass interposed. 



Would not these different alternations, produced in the relative 

 energy of the two images, by the interposition of a given screen, 

 be equally sensible to us, if our eyes lost the faculty of distin- 

 guishing colours, only retaining a perception of luminous in- 



