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every case 1 ). On the other hand: differences of symmetry in the 

 effects observed can only be possible, if they are present likewise 

 in the determining causes or circumstances. 



A pencil of polarised light travelling in the ether, has undoubtedly a 

 lower symmetry (Z)f ) than the ether itself (K&), which lower symmetry 

 is of course connected with the absence of certain symmetry-elements 

 in the luminous source from which the polarised light takes its origin. 



But the whole complex of light-phenomena (radiations) in the 

 ether, or in a crystal of calcite, etc., caused by this luminous source, 

 has certainly the same symmetry (Kg) as that of the ether, or a 

 higher symmetry (>g) than that of the calcite-crystal. 2 ) 



From this and analogous examples we can in general conclude 

 that a lack of symmetry-properties in the causes of physical pheno- 

 mena, can, (and as a matter of fact in by far the greater number 

 of cases will) be manifested also as a lack of symmetry-properties 

 of the effects produced, but that this need not always be the case. 

 From the absence of symmetry-properties in the effects observed, 

 however, it is certainly necessary to conclude that there is a similar 

 lack of symmetry-properties in the producing agents. 



In other words: the effects may occasionally have the same or a 

 higher symmetry than the causes, but the last cannot have a higher sym- 

 metry than the effects observed. 



It is, moreover, worth remarking in this connection, that symmetry- 

 properties which are present in all causes, and in all circumstances 

 governing a certain phenomenon, are necessarily always found in 

 the effects. However, we must always be sure that the number of 

 causes considered is really complete; evidently it is in many cases 

 hardly possible to get full assurance of this. 



1 ) The occurrence of the phenomena of pyro- and piezo-electricity in crystals 

 is dependent on the special symmetry-character of the medium ; the molecular 

 arrangement of it must, therefore, be taken into account as one o fthe "causes" 

 governing the physical phenomena mentioned above. On the contrary: light- 

 radiation may be arbitrarily produced in any crystalline medium under all circum- 

 stances; the possibility of its occurrence is here quite independent from the special 

 symmetry-properties of the medium itself. In the latter case, therefore, the 

 medium is not to be considered as a true physical "cause" in discussions of this 

 kind, as far as the possible manifestation of the phenomenon is concerned. 



2) The "image" / of the phenomenon of the propagation of rectilinear 

 polarised light in a calotte-crystal, can be represented by a rotation-ellipsoid 

 in every point P, with its axis of isotropy parallel to the trigonal axis of 

 this ditrigonal crystal. (See further on). 



