( 566 ) 



Readily cleavable parallel to a and c. 



The optical axial plane is jOlOj. Very faint, inclined dispersion: 

 ^^v\ double refraction negative. On c one optical axis emerges at 

 a small angle with the normal. 



Topic axial relation : x : H' : w =: 7.1503 : 5.3109 : 8.3556. 



Physics. — ''New observations concerning asymmetrical triplets''. ]iy 

 Prof. P. Zeeman. 



Asymmetry investigated by means of Fabry and Perot's method. 



1. In the second part of the paper "Magnetic resolution of spec- 

 tral lines and magnetic force" I ^) investigated, by means of a method, 

 which I called that of the non-uniform field, the asymmetry predicted 

 from theory by Voigt ') in the case the original line is resolved 

 into a triplet. 



A glance at Plate II of my paper immediately shoyvs that obser- 

 vation seems to confirm strikingly Voigt's theoretical result that 

 the component of the triplet towards the red is at a somewhat smaller 

 distance from the middle line than the one towards the violet. 



In order to exclude however all doubt as to the reality of this 

 experimental result I thought it desirable to continue my work in 

 a direction independent of Rowland's method. 



I have shown ') that the resolution of spectral lines by magnetic 

 forces can be investigated by means of the semi-silvered parallel 

 plates of Fabry and Perot. 



Using the special form of instrument in which the distance of the 

 silvered surfaces is constant, the étalon, we may yet choose between 

 two ways of comparison of the wavelengths of the centre line and 

 of the components, originating by the action of the magnetic field. 



Firstly we may measure, the intensity of the field being arbitrarily 

 chosen, the diameters of the interference rings. By combining only 

 measurements of rings originating from the same ring the calculation 

 becomes verj' simple ; for as shown in my last paper even a know- 

 ledge of the ordinal number of the rings then is unnecessary. 



2. We mav use however also the method of coincidences, regulating 



Ï) Zeeman. These Proceedings 30 November 1907. 



2) Voigt. Ann. d. Phys. 1. p. 376. 1900, see also the last paper by Voigt. 

 Physik. Zeitschrift 9. p. 122. 1908. 



3) Zeeman. These Proceedings 28 December 1907.. 



