546 HENKY A. EOWLAND 



Weight. Observer. D. 



o 



1 Angstrom, corrected by Thalen 5895-81 



2 Miiller & Kempf 5896-25 



2 Kurlbaum . 5895-90 



5 Peirce 5896-20 



10 Bell . ..5896-20 



Mean 5896-156 



As the relative values are more important for spectroscopic work 

 than the absolute, I take this value without further remark. It was 

 utilized as follows: 



1st. By the method of coincidences with the concave grating, the 

 wave-lengths of 14 more lines throughout the visible spectrum were 

 determined from this with great accuracy for primary standards. 



2d. The solar standards were measured from one end of the spectrum 

 to the other many times; and a curve of error drawn to correct to these 

 primary standards. 



3d. Flat gratings were also used. 



4th. Measurements of photographic plates from 10 to 19 inches long 

 were made. These plates had upon them two portions of the solar 

 spectrum of different orders. Thus the blue, violet and ultra violet 

 spectra were compared with the visible spectrum, giving many checks 

 on the first series of standards. 



5th. Measurements were made of photographic plates having the 

 solar spectrum in coincidence with metallic spectra, often of three 

 orders, thus giving the relative wave-lengths of three points in the 

 spectrum. 



Often the same line in the ultra violet had its wave-length deter- 

 mined by two different routes back to two different lines of the visible 

 spectrum. The agreement of these to y-j^ division of Angstrom in 

 nearly every case showed the accuracy of the work. 



6th. Finally, the important lines had from 10 to 20 measurements on 

 them, connecting them with their neighbors and many points in the 

 spectrum, both visible and invisible; and the mean values bound the 

 whole system together so intimately that no changes could be made in 

 any part without changing the whole. 



This unique way of working has resulted in a table of wave-lengths 

 from 2100 to 7700 whose accuracy might be estimated as follows: 



Distribute less than ^^ division of Angstrom properly throughout 



