41 

 TABLE OF STANDAKD WAVE-LENGTHS 



[Johns Hopkins University Circulars, No. 73, p. 69, 1889 ; Philosophical Magazine [5], 



XXVII, 479-484, 1889] 



In the ' American Journal of Science ' for March, 1887, and the ' Lon- 

 don, Dublin and Edinburgh Philosophical Magazine ' for the same 

 month, I have published a preliminary list of standards as far as could 

 be observed with the eye, with a few imperfectly observed by photog- 

 raphy, the whole being reduced to Bell's and Peirce's values for absolute 

 wave-lengths. Mr. Bell has continued his measurements and found a 

 slightly greater value for the absolute wave-length of the D line, and I 

 have reduced my standards to the new values. 



Nearly the whole list has been gone over again, especially at the ends 

 around the A line and in the ultra violet. The wave-lengths of the ultra 

 violet were obtained by photographing the coincidence with the lower 

 wave-lengths, a method which gives them nearly equal weight with 

 those of the visible spectrum. 



The full set of observations will be published hereafter, but the pres- 

 ent series of standards can be relied on for relative wave-lengths to -02 

 division of Angstrom in most cases, though it is possible some of them 

 may be out more than this amount, especially in the extreme red. 



As to the absolute wave-length, no further change will be necessary, 

 provided spectroscopists can agree to use that of my table, as has been 

 done by many of them. 



By the method of coincidences with the concave grating the wave- 

 lengths have been interwoven with each other throughout the whole 

 table so that no single figure could be changed without affecting many 

 others in entirely different portions of the spectrum. The principal dif- 

 ference from the preliminary table is in the reduction to the new abso- 

 lute wave-length by which the wave-lengths are about 1 in 80,000 larger 

 than the preliminary table. I hope this difference will not be felt by 

 those who have used the old table because measurements to less than A- 



o 



division of Angstrom are rare, the position of the lines of many metals 

 being unknown to a whole division of Angstrom. As the new map of 

 the spectrum has been made according to this new table, I see no further 

 reason for changing the table in the future. 



