with the Aid of a Grating. 



Table IV. 



Values of dX/dN, d0/ilN. e = 'G8 cm. 



431 



Spectrum lines... 



= 



B. 



D. 



E. 



F. 



G. 





Equation 22:* 



d\/dN = 





-•0055 



00 



+ 0081 



+ •0031 



cm. 



22 : 



d\/dN= 



- 0044 



-•0050 



00 



4- -0075 



+•0023 



cm. 



22 : 



d9/dN = 



- 14-6 



- 17-0 



00 



4-260 



+ 81 



rad. 



Thus 



Constants interpolated between D and G by /i=a+6X+c\ 2 , 

 where b= -00273, c= '0000197. 



dX 



dN N — efi cos R + eX(dfi/dX) /cos R ' 



(22) 



so that if dX/dN = c© , the maximum at the centres of 

 ellipses, the simultaneous effect at X' will be (as the mirror 

 has not moved) 



dX' 



X' 



dN 



e (jjl cos R — fjJ cos R') — el 

 \ 



dix 



X' d 



cos R dX cos 



R' dX'J 



If the centre of ellipses is at the E line the values of Table TV. 

 hold. The motion on the blue side of the E line is llms 

 larger than the simultaneous motion on the vellow side, 

 conformably with observation. 



12. Interferometry in Terms of Radial Motion. — Either by 

 direct observation or combining the equations (20) and (22) 

 for dX/dn and dX/dN, the usual equation for radial motion 



again results 



dN 

 dn 



X 



2' 



where N is the displacement of mirror per fringe. This 

 equation is best tested on an ordinary spectrometer by aid 

 of a thin compensator of microscope glass revolvable about 

 its axis and placed parallel to the mirror M. The change of 

 virtual thickness e' for a given small angle of incidence I 

 may then be written : 



71 ,sinRdR e' dl 2 lfii 2 1 



de 1 = e' rw^'o i — flr-2) nearly. 



cos-R 2 1— 1//A 



