96 



11. After the general remarks on the dependence of the sym- 

 metry-character of causes and effects in physical phenomena we 

 return to the consideration of some special symmetry-properties 

 of certain physical states and to the question, in what way several 

 simultaneously acting causes can cooperate as a resulting cause, 

 producing certain effects. l ) 



If a crystal of calcite is traversed by rectilinear polarised light, 

 and if we wish to give an exhaustive description of the way in which 

 the propagation of light-waves takes place therein, experience 

 teaches us that it is sufficient for this purpose, if we adopt as the 

 "image" of the phenomenon in every point P a rotation-ellipsoid 

 of certain dimensions and with its axis of isotropy parallel to the 

 ternary axis of the crystalline medium. The symmetry of the image 



/ is now, as already stated, Z)g , 

 while that of the crystalline me- 

 --VLV.T dium, as concluded from its 

 - - - molecular structure or from its 

 ____~^ cohesion-phenomena, is only that 



- _ __ _ of the group D. The last group 

 XT JT ~ is a sub-group of Z)g , a fact 

 to be remembered in what follows. 

 Fig. 95. In the same way, if we ask: 



what symmetry is to be attributed 



to the homogeneous electric field, as e. g. it may be produced 

 between two parallel, infinitely extended, condensor-plates, the 

 answer is, that we can attribute to it the symmetry of the group C , 

 the parallel lines of force of the field having the direction of the 

 axis of isotropy A^ . 



If now the last mentioned symmetry is given to the image / which 

 describes the physical state of every point P of the electric field, 

 the question may rise, whether the special symmetry of the image / 

 describing the physical state in every point P of the homogeneous 

 magnetic field be the same or perhaps another? 



Now, it is a wellknown fact that the action of a magnetic field 

 at each point P can be imagined to be produced by an electric current 

 of a definite direction, flowing in a circular circuit round P as its 

 centre, and with its plane perpendicular to the lines of force of the 



J ) Cf. P. Curie, Journal de Physique, (3), 3, 407, (1894); Bull, de la Soc. 

 Min., 7, 89, 418, (1884). 



