518 HENRY A. ROWLAND 



No attempt has been made to reduce the figures to a vacuum as the 

 index of refraction of air is imperfectly known, but this should be done 

 where numerical relations of time period are desired. 



In the column giving the weight, the primary standards are marked 

 8 and the other numbers give the number of separate determination of 

 the wave-length and thus, to some extent, the weight. 



Many of these standards are double lines and some of them have 

 faint components near them, which makes the accuracy of setting 

 smaller. This is specially the case when this component is an 

 atmospheric line whose intensity changes with the altitude of the sun. 

 The principal doubles are marked with d, but the examination has not 

 been completed yet, especially at the red end of the spectrum. 



[A table of the standard wave-lengths is given on p. 78 J. H. U. Circ., 

 but is omitted in this volume.] 



