Institute of France. 38> 



well as with thin plates on one and the same luminous ray. 

 V/hen these plates are of the same nature, the opposition is al- 

 ways effected by crossing at riglit angles their axes of double 

 refraction. But when they are of a different nature, in certain 

 cases the axes must be crossed, and in others they must be ren- 

 dered parallel. This last case takes place, for instance, when 

 \V8 combine the needles of beryl with those of quartz. When 

 the axes of these two sal)stances are placed in the same way re- 

 lative to a polarized ray, the impressions which they communi- 

 cate to it are such, that if they are successive they dejstroy each 

 other, and on the contrary they are continued and added toge- 

 ther if the axes are crossed at riglit angles : this is precisely the 

 ii)verse of what we observe when we qombine two plates taken 

 from on.e and the same metal. Thus, in that kind of magnetiza- 

 tion with which crystals act on the luminous particles which 

 pass through them, we must distinguish two modes of impres- 

 sion different and opposite to each other, like the vitreous and 

 resinous electricities, or the two boreal and austral magnctism.s. 

 We may call them the quartzous and beryllated polarizations. 

 The following is a list of some substances which belong to one 

 or other of these denominations ; 



Quartzous polarization: Rock crystal, sulphated lime, sul- 

 phated barytes, topaz. 



Beryllated polarization : Rhomboidal carbonated lime, arra- 

 gonite, phosphated lime, beryl, tourmaline. 



When we combine together two crystals the polarization of 

 which is of the same nature, we must cross their axes in order 

 to obtain the differences of their actions ; and, on the contrary, 

 we must render them parallel if their polarizations are different. 

 We see that the primitive form of a crystal has no evident rela- 

 tion with the kind of polarization which it exercises, any more 

 than it has with the electrical properties of the minerals. 



By studying the action of tourmaline upon light, M. Biot re- 

 cognised in it the singular property of having the double refrac- 

 tion when it is thin, and the simple refraction when it is thick. 

 Ill order to make these phaenomena clear, he polished the in- 

 clined faces of a large piece of tourmaline, so as to form with it 

 a prism, of which the section was parallel to the axis of the 

 needle, which is also that of the primitive rhomboid, if vve 

 look at the flame of a taper through this prism, directing the 

 visual ray into the thinnest part, we see two images of a lustre 

 sensibly equal, one of which, ordinary, is polarized in the direc- 

 tion of the axis of the tourmaline ; and the second, extraordi- 

 nary, is so in a direction perpendicular to this axis, But in pro- 

 j)ortion as we carry the visual ray into the thickest part of the 

 prism, the ordinary image is weakened, and finally disappears 



Vr)|. 1.->. No. 'iOS. /i;«y lSir». Bb entirely: 



