571 



TABLE 617.— IRON SECONDARY STANDARDS OF WAVELENGTH 



IN ANGSTROMS 



Iron tertiary standards of wavelength. — The iron tertiary standards derived from 

 diffraction-grating interpolation between secondary standards, and formerly adopted, 18 * have 

 all been superseded by interferometer, or grating interpolated, values published in the 

 M.I.T. Wavelength Tables (John Wiley & Sons, New York, 1939). 



Extreme ultraviolet standards of wavelength. — Provisional standards of wave- 

 length in the extreme ultraviolet, measured relative to secondary and tertiary iron stand- 

 ards in overlapping: spectral orders, have been published ; 19 ° they include 57 carbon lines 

 (1930.900 to 858.091 A), 23 nitrogen lines (1745.246 to 775.966 A), 25 oxygen lines 

 (1306.038 to 580.974 A), and 10 argon lines (1066.660 to 871.099 A). 



Standard solar wavelengths.— The International Astronomical Union 101 has adopted 

 7-figure standards of wavelength in the solar spectrum when two or more accordant values 

 have been reported. These values have resulted mainly from interferometer measurements 

 of solar-absorption wavelengths relative to neon or to iron secondary standards. The 

 standards represent integrated solar light, are corrected for Doppler-Fizeau displacement, 

 and are valid for standard air at 15°C and 760 mmHg pressure. In the long-wave region 

 many of the solar spectrum standards originate in the terrestrial atmosphere as absorption 

 by oxvgen or water vapor. 



In Table 618 the -f sign following the symbol of an element indicates ionization; a sym- 

 bol like Fe — , solar line too strong to be due to iron alone ; Fe, Co, coincidences of like 

 order ; Fe Co, coincidence closer for preceding element ; Fe — Co, Fe wavelength smaller, 

 Co larger than solar line ; an italicized element indicates a more prominent contribution 

 and boldface a decidedly predominant element. 



188-191 p or references, see p. 578. 



SMITHSONIAN PHYSICAL TABLES 



