532 TABLE 553.— INDEX OF REFRACTION OF AIR (15°C, 76cmHg) 



Corrections for reducing wavelengths and frequencies in air (15°C, 76 cmHg) to vacuo 



The indices were computed from the Cauchy formula (n — 1)10 7 = 2726.43 -f 12.288/(\ 2 X 

 10" 8 ) + 0.3555/(\* X 10" 10 ). For 0°C and 76 cmHg the constants of the equation become 2875.66, 

 13.412 and 0.3777 respectively, and for 30°C and 76 cmHg 2589.72, 12.259 and 0.2576. Sellmeier's 

 formula for but one absorption band closely fits the observations: n 2 = 1 + 0.00057378X 2 /O 2 — 

 595260). If n— 1 were strictly proportional to the density, then (n — l)o/(n — \)t would equal 

 1 _(_ a t where a should be 0.00367. The following values of o were found to hold : 



X 0.85/* 0.75m 0.65m 0.55m 0.45m 0.35m 0.25m 



a 0.003672 0.003674 0.003678 0.003685 0.003700 0.003738 0.003872 



The indices are for dry air (0.05 ± % C0 2 ). Corrections to reduce to dry air the indices for 

 moist air may be made for any wavelength by Lorenz's formula, + 0.000041 (m/760), where m 

 is the vapor pressure in mm. The corresponding frequencies in waves per cm and the corrections 

 to reduce wavelengths and frequencies in air at 15"C and 76 cmHg pressure to vacuo are given. 

 E.g., a light wave of 5000 angstroms in dry air at 15°C, 76 cmHg becomes 5001.391 A in vacuo; 

 a frequency of 20,000 waves per cm correspondingly becomes 19994.44. 



SMITHSONIAN PHYSICAL TABLES 



