293 



Table 349, 



INDEX OF REFRACTION. 



TABLE 349. — Index of Refraction of Air (15°C, 76 cm). 



Corrections for reducing wave-lengths and frequencies in air (75° C, 76 cm) to vacuo. 



The indices were computed from the Cauchy formula (» — r)io' = 2726.43 + i2.288/(\2X io~') +0.3553/ 



(h* X io~i6). For 0° C and 76 cm the constants of the equation become 2875.66, 13.412 and 0,3777 respectively, and 

 or 30° C and 76 cm, 2589.72, 12.259 and 0.2576. Sellmeier's formula for but one absorption band closely fits the 

 observations: »2 = i -|- o.ooo57378XV(^^ — 595260). If w — i were strictly proportional to the density, then (« — 1)0/ 

 (» — i)« would equal 1 + a/ where a should be 0.00367. The following values of a were found to hold: 



X 0.85/x 0.7SM 0.65M 0.55/U 0.45^1 0.35^ 0.25^ 



a 0.003672 0.003674 0.003678 0.003685 0.003700 0.003738 0.003872 



The indices are for dry air (0.05 ± % CO2). Corrections to the indices for water vapor may be made for any wave- 

 length 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 wave-lengths and frequencies in air at 15° C and 76 cm pressure to vacuo 

 are given. E.g., a light wave of 5000 Angstroms in dry air at 15" C, 76 cm becomes 5001.391 A in vacuo; a frequency 

 of 20,000 waves per cm correspondingly becomes 19994.44. Meggers and Peters, Bui. Bureau of Standards, 14, p. 731, 

 1918. 



Smithsonian Tables. 



