OF ARTS AND SCIENCES. 291 



being an obstacle, can be turned to great advantage. In the absolute 

 measurements of tbe wave-length of light, the spectrometer method 

 with eye observation and with a micrometer is unquestionably more 

 accurate than any photographic method. We have in this determina- 

 tion to deal with comparatively large quantities, and with well defined 

 directions, which can be made to coincide with optical axes of the 

 instrument ; this is not the case, however, with the majority of the 

 spectral lines on a photographic plate placed in a camera, which 

 replaces the observing telescope of the spectrometer. The photograph 

 contains possible errors, and any shifting or movement of the spec- 

 trometer circle to determine intervals on the photographic plate is apt 

 to introduce other errors. 



The ideal arrangement would seem, therefore, to be a photographic 

 apparatus which should remain in focus for all the spectra of the dif- 

 ferent orders, in which distances between successive lines on the photo- 

 graphs of the spectra should be closely proportional to wave-lengths, 

 so that, the constant being known for a certain position of the sensitive 

 plate, the wave-lengths can be determined by simple linear measure- 

 ment. Moreover, it is desirable, as we have said, that the underlying 

 spectra should be brought to the same focus as the overlying; for by 

 this means we can compare the wave-lengths of lines in the spectra of 

 different orders, and halve our errors. It is true that some confusion 

 results from having the metallic lines in the spectra of different orders 

 photographed upon the same plate ; but a little experience enables one 

 to separate the lines with comparative ease, and the gain in accuracy 

 compensates for the additional trouble. 



The apparatus which best answers the requisitions we have pointed 

 out is that of the concave grating of Rowland, with its peculiar mount- 

 ing, which has been fully described in the American Journal of 

 Science, Vol. XXVI., 1883, p. 87. 



Objects of the Present Investigation. 



The conclusion of the work of the Committee of the British Asso- 

 ciation on the tabulation of metallic spectra seemed to us to require a 

 survey of the work, which must be done in the future in order to per- 

 fect and correct the work of the past. We have therefore examined 

 the tables given by the committee in order to see what lacunar could 

 be supplied, and to point out the directions for routine work which 

 may afford material for future generalizations. In the pursuance of 

 this work, we have been compelled to examine the accuracy of meas- 

 urements of wave-lengths hitherto made in the ultra violet. With 



