SPECTROSCOPIC PHOTOGRAPHY 



817 



When diaphragms are used at or before the slit, one should remember that the 

 lower part of a spectrum line corresponds to the upper part of the slit and vice versa, 

 since a spectrograph produces inverted real images of its slit. 



Identification of Spectrum Lines. — A spectrum line is best identified by its wave- 

 length, which can be deduced from its position in the spectrum and under proper 

 conditions can be determined to one part in several million. Most wavelength deter- 

 minations are made photogi-aphically and involve first the determination of the relation 

 between the wavelengths X of known lines and the distances I of these lines from some 

 fixed point on the plate. This relation, when plotted graphically, gives the dispersion 

 curve of the plate, which is seldom a straight line. The dispersion curve can be plotted 

 graphically or determined mathematically without plotting, and from it the wave- 

 lengths of unknown lines can be deduced by interpolation of their observed positions. 



Wavelengths can be determined from theoretical dispersion formulas, but so many 

 easily identifiable lines have now been precisely measured that these formulas are 

 seldom used except in special cases. 



Standards of Wavelength. — The international angstrom unit is defined as a length 

 equal to the wavelength of a certain red line in the cadmium spectrum divided by 

 6438.4696. In other words, this red line, the so-called "primary standard" of wave- 



Section throughyA 

 Fig. 10. — Diaphragms for shortening the lengths of spectrum lines in making wavelength 



determinations. 



length, has by definition the length 6438.4696 A. A number of "secondary standards " 

 have been measured in terms of this line by means of interferometers. Wavelengths 

 of the secondary standards are given in the Transactions of the International Astro- 

 nomical Union, 3, 86 (1928); and International Critical Tables, Vol. 5, p. 275. 



The secondary standards are somewhat far apart in the spectrum for convenient 

 use, and to fill the gaps between them, "tertiary standards" have been carefully 

 measured by means of gratings and interferometers. These "working standards" 

 of wavelength, as adopted by the International Astronomical Union, are given in the 

 International Critical Tables, Vol. 5, p. 275. 



Wavelength Tables. — For routine spectrographic work sufficiently precise wave- 

 length values can be obtained from the "Handbuch der Spectroskopie," by H. Kayser, 

 an encyclopedic work of eight volumes in German which lists more than 120,000 

 lines. More convenient is the shorter list of "Tabelle der Hauptlinien der Elemente," 

 by Kayser, which gives wavelengths for approximately 19,000 of the more important 

 spectral lines of the elements, and the M.I.T. wavelength tables which list wave- 

 lengths and intensities for the 110,000 most important lines between 10,000 and 

 2,000 A. Wavelengths determined before 1911 are usually on the old Rowland 

 scale which has been superseded by the International Scale, and require corrections 

 varying between 0.12 A. at 2950 A. and 0.28 A. at 7700 A. 



Wavelength Determinations. — When an unknown spectrum is photographed, 

 provision should be made to photograph a known spectrum and a spectrum containing 



