112 KNOBLAUCH ON THE TRANSMISSION 



axis of the brown rock-crystal, beryl or tourmaline, when their 

 plane of polarization forms an angle of 90° with the axis of the 

 crystal, are transmitted in quite different proportions from those 

 observed when the line of transmission is parallel to the axis. 



The rays traverse the crystal in all directions alike when their 

 plane of polarization coincides with the axis of the crystal. 



3. In the first case the greatest qualitative differences manifest 

 themselves, in the second case no such differences have an 

 existence. 



4. When the rays are transmitted along the axis, no differences 

 are observed either as regards quality or quantity, whatever may 

 be the position of the plane of polarization. 



5. Compared among themselves, the different directions per- 

 pendicnlar to the axis exhibit no differences of transmission in 

 the three crystals above-mentioned. 



When we reflect that (according to former investigations) the 

 natural calorific rays, on passing through the crystals perpendi- 

 cular to their axes, are decomposed by double refraction into two 

 groups, and so polarized that the plane of polarization of the one 

 group coincides with the axis of the crystal, while that of the 

 other group is perpendicular to the same ; further, that this does 

 not take place when the direction of transmission is along the 

 axis ; this, in connexion with what has gone before, explains the 

 deportment observed in the case of the natural calorific rays. 



Those rays which proceed parallel to the axis of the crystal 

 undergo a certain absorption which depends upon the nature of 

 the crystals, and determines the quantity and character of the 

 transmitted rays, — determines, for instance, their deportment 

 towards diathermanous bodies. In passing perpendicular to the 

 axis of the crystal, the two groups of rays are absorbed in dif- 

 ferent proportions. Those whose plane of polarization coincides 

 with the crystallographic axis suffer the same absorption as the 

 rays which pass along the axis ; the other rays, however, will be 

 absorbed either in a greater degree, as in the case of brown rock- 

 crystal and beryl, or in a less degree, as in the case of tourmaline. 

 Further, as this absorption is elective, that is to say, directed in 

 different degrees towards the different rays of one and the same 

 group, the composition of such a group will be modified, and 

 hence its capacity to penetrate a diathermanous body ^\ill be 



