262 BELL SYSTEM TECHNICAL JOURNAL 



whence 



and if e is small 



Na sin e 



H = - 



tan R 



From Eq. 2.13, e = 0.530 tan^ d tan R radians, and putting this in the equa- 

 tion for H we find that the correction for ring eccentricity approximately 

 annuls the correction for refraction if 



H = - .530 tan2 d 



For d = 10° this gives H = - .0255, that is, a fluid index of 1.5207. 



