293 



INDEX OF REFRACTION. 

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



Corrections for reducing -wave-lengths and frequencies in air (15 C, 76 cm) to vacua. 



The indices were computed from the Cauchy formula (n i)io 7 = 2726.43 + i2.288/(A*X io~ 8 ) +0.35557 

 (A 4 X icr 16 ). For o C and 76 cm the constants of the equation become 2875.66, 13.412 and 0.3777 respectively, and 

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

 observations: n 2 = i + 0.0005 73 78A 2 /(X 2 595260). If n i were strictly proportional to the density, then (n-i V 

 ( i)t would equal i + at where a should be 0.00367. The following values of a were found to hold: 



A o.Ss/i o.75/i 0.65/1 0.55/1 0.45/4 0.35/1 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 % CO 2 ). Corrections to the indices for water vapor may be made for any wave- 

 length by Lorenz's formula, + 0.000041 (m/jfo), 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. 



