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tromagnetic field", the general relations between electric and mag- 

 netic quantities shall preserve their validity, and that, therefore, 

 the said mirror-image shall also have the function of a possible electro- 

 magnetic system, then we have to decide which of the two following 

 standpoints we wish to adopt: 



a. Either in the mirror-image we can take as electric vectors 

 (electric force, current, dielectric polarisation) the mirror- images 

 of the original electric vectors, and as magnetic vectors (magnetic 

 force, magnetic induction, etc.) the in versed mirror- images of the 

 original magnetic vectors; 



b. Or in the mirror-image we can take as magnetic vectors the 

 mirror-images of the original magnetic vectors, and as electric 

 vectors the inversed mirror-images of the electric vectors in the 

 original electromagnetic field. 



In fixing our choice in the way first mentioned, we have in a 

 homogeneous electric field symmetry-planes passing through the 

 lines of force, in the magnetic field, however, a single symmetry-plane 

 perpendicular to the lines of force. But in fixing our choice in the 

 second way, the functions of the electric and magnetic fields are 

 exactly interchanged. 



Now there are "mechanical" theories of the electromagnetic 

 field, which are founded on the first conception; but there are also 

 theories which start from the second point of view. However, if 

 we should wish to describe the electromagnetic phenomena in 

 certain cases by the motion of ions or electrons, which has many 

 and wellknown advantages, the first standpoint is certainly 

 more convenient. These motions then, and the moving ions or 

 electrons themselves can be looked upon as reflected in a plane, 

 and it might be imagined that electric charges are attributed to 

 the "reflected ions" or electrons with the same algebraic sign as 

 they have in the original electromagnetic field. In this way a 

 description of the phenomena in the "mirror-image" will be possible 

 just in the same way as if we were dealing with the original 

 field; and the mirror-image is thus in truth a "possible" electro- 

 magnetic system, fulfilling the above mentioned condition of 

 the preservation of the general relations between the electric and 

 magnetic parameters. 



From this it will now be clear that the symmetries attributed 

 to physical phenomena are really relative symmetries, determined 

 by the general relations between the different natural phenomena 



