( 4as) 



Physics. — Dr. J. J. Hallo: ''Thr rnlue of sonic /ita<//trfo-optic 

 constants." (Communicated by Prof. P. Zkkman). 



Ill my doctoral thesis, on The inat/ni't/'r fotrtfion of the plane of 

 polarisation in the neighhourhood of an ahsorption-t)and ^), I Xvaxq cal- 

 culated the values wiiich three of the constants, occurring in Voujt's 

 theory of ma»neto-optic phenomena, assume in a particular case. I did 

 not then know as yet, tiiat Dkidk had already tried — in his Lfhrhuch 

 der Optil- (Lei])zig, 1900) — to make some estimate as to llie order 

 of magnitude of a constant r, introduced l\y him, Ayhicli is connected 

 in a simple manner with one of the constants of Voigt's theory, of 

 which I have determined the \alue. Therefoi'e 1 beg to be alloweil 

 to mention here my results and those of Dkuue, and to examine in 

 how far these results agree. 



If ^5 S^, 5 i^i'e the components of the electric polarisation in some 

 medium, Voigt assumes that every one of these componeids exists 

 of a part A', Y, Z, rehiting to the free ether, and a series of other 

 parts 9c/(, ^i,, 3ii, indicating the state of the ponderable matter. He 

 therefore assumes : 



c£ r= X -j- -2" dij,, etc. 



A representation of the ])henomena of selecliye absorption, in which 

 the inlhience of a magnelic field with components ^I, />*, C is also 

 taken into account, is gained when the sets of \'ector-compoiients 

 ^4, 7i, 6' are subjected to the conditions: 



i/. + ../. ^ + h ^ + .7. [i -^-B --J = .;, A, ete. 



The constant i)^ a[)pears to be equal to T/i74::t^, if t/, is the \ibi'atoiy 

 period of a free vibration of tiie al)S()i'bing medium; 1 ha\e deri\ed 

 the values of the constants «/„ Cj, and f/j for the line I)., from the 

 results of my measurements in a ))articular case (for a ihxme wiiich 

 contained yery much sodium); the values 1 have found are {vide 

 p. 85 of my thesis) : 



an — 1 . 10-20 . 

 Ck r= 0,2. 10-2.3 . 

 Bk= 1,^0. 10-8 . 

 The constants a^ and e/j depend on the density of the sodium- 

 vapour in the ihime, tlie constant cu does not so far as we know. 

 The data whicli seryed for the calculation of these constants are 

 the following: ah was calculated from the width of the absorption- 



1) Amsterdam, 1902. 



