172 M. MELbONI ON THE POLARIZATION OF HEAT. 



of lime or mica, and afterwartls fall upon a second surface of 

 glass at an equal inclination of 30" 25', the latter reflects the in- 

 cident light in a larger or smaller quantity, according to the 

 positions which the principal section of the ciystallized lamina 

 and the jjlane of the second reflection aflfect, in relation to the 

 plane of primitive reflection, in which the ray of light is first 

 polarized. 



Let us consider the two mirrors independently of the crystal- 

 lized lamina. If we first make their planes of reflection to coincide, 

 and afterwards place them perpendicularly, in the first position 

 we shall obtain the maximum of reflected light, in the second 

 the minimum. The effect is unaltered if the doubly refracting 

 lamina be interposed between the two mirrors, after tracing 

 upon its edges the direction of the principal section, and bi'ing- 

 ing it parallel or perpendicular to the primitive plane of reflec- 

 tion ; the proportions of light reflected by the second mirror 

 remain the same in each case; hence the denomination of 

 neutral axes, given to these two directions of the lamina. But 

 if the principal section or its perpendicular be inclined in such 

 a manner that one of them forms an angle of 45° with the plane 

 of primitive polarization, there is a very considerable alteration 

 in the reflection of the second muTor ; the maximum of reflected 

 light is diminished, the minimum increased ; and the diminu- 

 tion of intensity produced in the first case, when the planes of 

 reflection are parallel, is found to be precisely equal to the aug- 

 mentation which occurs in the second case, when the planes of 

 reflection are perpendicular. 



These variations of intensity, caused by the particular posi- 

 tion of the principal section of the bi-refracting crystal, in rela- 

 tion to the plane of primitive polarization, require for their pro- 

 duction a certain thickness according to its nature, but always 

 extremely small, of the lamina interposed : they are besides ac- 

 companied by a brilliant coloration, which ceases also at certain 

 limits of thickness, equally dependent upon the quality of the 

 crystal interposed. We here lay aside the subject of colours, 

 and shall only consider the intensity, which always follows the 

 law enunciated whether the colours be perceived or not, the re- 

 flected light in the latter case appearing perfectly white, as hap- 

 pens with plates of sulphate of lime rather more than a demi- 

 milhmetre in thickness, and with plates of mica nearly twice as 

 thick. 



