( ^^- ) 



iiirlepciirleiit of tlio aocnr.ite Njiliie ol" llic lliickiicss of tho |il;itc 

 of air. 



Of course tlie posilioii ol' llie new riiij>'s l»el\ve;'ii tlie riiii^s ;.„ will, 

 willi a ü;i\'eii xaliic of llie ma{iiietic force, he (leterj)iiiie(l Itv the 

 tiiiekiiess of (iie plale of air and what might i)e called "the sensihi- 

 lity" of llie .system of rings lo mamielic forces will increase with 

 the thickness of the plate of air. A linut of this sensihility is (oflen 

 U)o soon) attained hy the elVeclive width of the specli'al lines nnder 

 consideration. 



In some cases it will he desiral)lc to sclccl for measni-ement rinus 

 ditfereni from the ihi'ee specilied ones. There ai'e no diflicnllies 

 ahoul Ihe signilicance of /^ it always means the oi'dinal nnmher of 

 the measin-ed ring. 



However if /''„ ditfers fronr I\ oi- I^-, theii- values mnsi he known 

 for the calcnhition according to (2) and (3). 



4. Besides the simplification resulting from e(jnalion (4) there is 



still airother one to he considei'ed in the in\estigalion of the radiation 



in a magnetic field. 



A 

 I mean that the cpiaidity e^ip-^, the optical thickness of the plate 



of air may he treated as an ahsolnte constant. 



Oi'dinarily this thickness depends upon ).. In conseqnence of tlh» 

 change of phase hy i-efiection upon the silver, which varies with 

 wavelength, tlie comparison of different colonred systems of rings 

 finally requires a knowdedge of the optical thickness for each separate 

 colour. 



It is clear that in the apphcalion to the subject JiowMuidei- rexiew 

 only systems of rings coi'res[)onding to rays diffei-ijig extremely little 

 in wavelength are considered, hence the \ariation of thickness uith 

 waveleiigth needs not to be taken into account. 



5. Figin-es I and "2 may give an idea of the aspect of the magnetic 

 resolntion of the spectral lines observed l)y means of the metliod of 

 P'abky and Pkrot. They are about sixfold enlargements of negatives 

 taken with an étalon uath an intei'val of nearly 5 m.m. i)etween 

 the plates. The source of light in the magnetic field was a small 

 vacuum tube chai-ged with mercni-y. The order of interference at 

 the centre for the mercury line 5791 is at 16' about 17265,7. 



The system of rings was formed in the focal plane of a small 

 achromatic lens of 18 m.nr. apei-ture and of 12 cm. focns. Its 

 focal [tlane coincides exactly with the [)lane of the slit of a small 



