ADAMS: RAYLEIGH-ZEISS INTERFEROMETER 275 



Therefore 



Eliminating dp by formula (10) we obtain finally 



(11) iV(- iVx= ^ [(5i -B,)l- qP:^ 



This expression gives the position of the achromatic fringe 

 relative to the central fringe for any wave-length X. Now if 

 by the combined effect of the change in concentration of the 

 solution and of movement of the compensator the position of 

 the fringes for wave-length X remains unchanged — that is, if, 

 as in actual practice, the solution has shifted these fringes Nt 

 fringes to the left and the compensator an equal number to the 

 right — then iV^ = 0; moreover 



Nt = — and obviously must also = - 



Combining these relations with (11) we have 



It will be convenient to take for X that wave-length which cor- 

 responds to maximum luminosity in the spectrum of the light 

 source (a tungsten lamp) ; this was found to be X = 0.58. Now 

 since m- - Uc. = B [l/(.486)2 - l/(.656)2] ^ 1,91 b, it follows 

 that 



B 3" 



Tin- I 1.91 



where /3" is the ordinary relative dispersion of the glass plate, 

 and similarly if we define the dispersive power^ ((3') 



^ It is to be noted that /8' is not the same as the difference in relative disper- 

 sion of the solution and water; i.e., 



riiu — 1 n2D — 1 



is not equal to (j'f-j'c)/»'d. Values of 0' and 0" can be found, or calculated 

 from data, in tables of constants. 



