( -A^l ) 



main objection is doi-ivcd tVoui the great loss of intensity in the 

 a|»|»aratus of Fahhy anil Pkhot. Tlic |)i'esent papoi- proves tliat this 

 oi)je('lion is not insuperable. 



2. Of the two forms in which the method of tlie parallel plates 

 is employed, the simplest, also reciniring the least costly apparatus, 

 has been nsed for the actual measurement of wave-lengths by Fabry 

 and Pkrot^), lord liAYLKum '^) and Evershkim ^). This form of instru- 

 ment called ('id/on, i also have preferred. The distance of the silvered 

 surfaces is here constant. The glass plates are held up to rounded 

 distance-pieces (made of steel), by adjnstable s|)rings, which permit 

 to regulate the pressure. By variation of the pressure the steel and 

 the glass can be deformed in an extremely small degree and the 

 accurate parallelism of the glass plates effected ; the parallelism being 

 secured already very apj)roximately beforehand by the accuracy or 

 finish of the distance-pieces. 



3, The theory of the comparison of wave-lengths by means oi 

 this apparatus is simple, and has been given by Fabry and Perot. 

 We will apply it to the magnetic resolution of spectral lines and 

 especially to the most simple case, the division into a triplet. 



Ijet P.(, be the wave-length of the original line (afterwards there- 

 fore the middle line of the triplet). To this corresponds a system of 

 rings; let P^ be the ordinal nnmber of the first from the centre. 

 The ordinal number at the centre j)^ is then the integer numbei' i-^^, 

 augmented with a fraction f^, hence l>o^= Po -\- ^a- Ordinarily 



The diameter of a ring increases with ?. I^et e be the thickness 

 of the plate of aii-, the oi'der of interference ar the centre is 



y>y =r — . At an angle l with the normal to the [)late the order 



of intei'ference is p^ cos i. 



If jCg be the angular diameter of the ring ]\, then we ha\'e, 



observing in the focal |)lanc of a lens, ^>q ctfó- - = i^o. Develojiing the 



cosinns 



>) Fabry et Perot, Ann. de Chim. et de Pliys. T. 25, Janvier 1002. G.R. 27 

 Mars 1904. Fabry et Buisson, G. R. IG Jaillet 1900. Barnes AsliopliysicalJoiunal. 

 Vol. 19. p. 190. 1904. 



2) Lord Rayleigh, Phil. Mag. Vol. 11, p. 685, .1900. 



3) Eversheim, Zeitschr. f. wissenscliafll. Photograpliie, Band 5 p. 152, 1907. 



