( 567 ) 



the magnetu' force in siieli a manner that a rijig which expands by 

 increasing magnetic intensity coincides with a contracting ring. 



The rings corresponding to components towards the red then 

 coincide vvitii rings due to components towards the violet side of the 

 spectrnm. Tlie intensity of tlie coinciding rings is then only slightly 

 inferior to that of the ujimodified one, a circumstance favourable to 

 the accuracy of the measurements. 



Let ).^ be the wavelength of the middle component of the triplet, 

 7.,. that of the component towards the YQi\, ?.,, that of the component 

 towards the violet then we may perform the calculation, ignoring t"he 

 value of the ordinal nundiers of the rings, by the following procedure. 



Let Po, Pr, F„ be the ordinal nnmbers of rings with angular 

 diameters x^, Xr, x^ then we have in general : 



If the magnetic force is increasing a contracting ring corresponds 

 to 7.,. , an expanding one to 7.^. As I remarked on a former occasion 

 we can put in the case of radiation in a magnetic Held P^ = P, or 

 P^ = P„ if only rings 2,- and X^ are considered, which originate from 

 the same ring A^, . 



Hence in applying the method of coincidences the simplest 

 procedure is to consider the ring formed by superposition of two 

 othei- rings, once as' a ring A,, derived from a smaller ring X^, and 

 again as a ring X,. derived from a larger one X^ . 



By measuring three rings viz. the one due to the coincidence 

 of the rings X,. and X,. (diameter x,: =z x = x^), further the larger 

 ring with diameter x^ and tinally the smaller one with diameter x'^, 

 the result may be found by the simple formulae : 



;., =: ;. f 1 + -f _ - 



and 



3. Using an etalon, with an interval of nearly 5 mm. between 

 the plates, I have made by means of the method of coincidences 

 some measurements of the magnetic resolution of the yellow mercury 

 Unes 579J and 5770. 



39 



Proceedings Royal Acad. Amsterdam. Vol. X. 



