FALSE SPECTRA FROM THE ROWLAND CONCAVE 



GRATING. 



By Theodore Lyman. 



Presented October 10, 1900, by W. C. Sabine. Received October 26, 1900. 



It is proposed to show in the following paper that among the spectra 

 formed by the Rowland concave gratings there are spectra not accounted 

 for by the ordinary theory of the grating ; that such spectra are common, 

 and at times fairly strong and of excellent definition ; that these spectra 

 are diffraction spectra, of much less dispersion than the ordinarily recog- 

 nized spectra, and that the errors of ruling to which they are due are not 

 local, but general to the whole surface of the grating. Finally, it is pro- 

 posed to explain an experimental method by which these false lines can be 

 sorted out from the regular and calculable overlapping spectra. These 

 lines are especially dangerous in series spectra work, giving a some- 

 what systematic reproduction of strong lines and groups, which reproduc- 

 tions in actual vibration frequencies do not exist. There is probability 

 and some evidence that such errors have been committed in the past, and 

 it was in the presence of this danger that the false spectra were here 

 discovered. 



In the year 1893, Schumann showed that the ultra-violet spectrum 

 could be extended to the neighborhood of 1000 jx. He used a spectroscope 

 fitted with a fluorite prism and fluorite lenses. The apparatus was so 

 constructed that it could be enclosed in a case from which the air could 

 be exhausted. According to his investigation, it was chiefly the absorp- 

 tion of the air for light of very short wave lengths that had prevented 

 other investigators from extending the spectrum below wave length 

 1800 ft. Owing to the nature of a prism spectrum, it was difficult for 

 Schumann to determine accurately the wave lengths of the lines which 

 he discovered. Since his paper no attempt seems to have been made to 

 measure these lines. It was in an effort to measure these wave lengths 

 that the false spectra above referred to were first observed. 



It seemed probable that if a concave grating could be used to obtain 

 these ultra-violet lines, an accurate determination of their wave length 

 vol. xxxvi. — 16 



