100 M. MELLONI ON THE POLARIZATION OF HEAT. 



diaphanous substance, in a direction more and more inclined 

 upon the plane of the lamina, it will be found gradually to dimi- 

 nish in intensity, in proportion to the increasing obliquity. It 

 may readily be conceived that such would be the case ; because 

 the rays that fall obliquely upon the lamina traverse a greater 

 thickness of glass than those which arrive in a direction more 

 nearly approaching the perpendicular, and suffer consequently a 

 greater absorption. But even if the matter of which the lamina 

 is composed were perfectly Hmpid, and admitted the passage of 

 all the light which penetrates into the interior, at any incidence 

 whatever, the decrease of intensity corresponding to the increa- 

 sed inchnation would still be obsen^able, because the luminous 

 rays undergo a partial reflection at the two surfaces of the lamina, 

 which is at first feeble and sensibly constant, for angles of from 30° 

 40° around the normal, but which is rapidly augmented at in- 

 creased inclinations, so that the pencil transmitted in a very ob- 

 lique direction to the surfaces of the lamina, loses a very great 

 portion of its intensity, solely on account of the reflection. 



The same results are produced Avith two or several successive 

 laminae ; but when the number is increased to about thirty, and 

 beyond that number, the effects produced are very different. 



If, for instance, a pile composed of forty or fifty plates of glass 

 superposed, be held, first perpendicularly to the incident rays, 

 and afterwards gradually inclined upon them, the feeble light 

 transmitted under the normal incidence, instead of being chmi- 

 nished by an increase of obliquity, becomes, on the contrary, 

 more and more vivid and brilliant, up to a certain inchnation ; it 

 then loses by degrees the intensity acquired, and, lastly, becomes 

 extinguished, when the rays by an excess of obliquity can no 

 longer penetrate into the A'itreous matter. Now, the angle at 

 which the pencil transmitted attains its maximum intensity, is 

 precisely that at which light is completely polarized by reflection. 

 This singular deviation from the ordinary laws of transmission, 

 is attributable, therefore, to a phaenomenon of polarization. Sup- 

 pose, first, for example, the pile inclined 35°*25', the value of the 

 angle at which light is completely polarized by reflection upon 

 glass : the refracted rays at this inclination will be strongly po- 

 lai'ized at a certain depth of the pile ; for we have seen that 

 light, as well as heat, is susceptible of complete polarization by 

 refi'action, at any angle whatever, if the laminae traversed be suf- 

 ficiently numerous. We also know that the plane of jwlariza- 



