60 IOWA ACADEMY OF SCIENCE Vol. XXV, 191» 



was exposed in a small apparatus and the measurement is there- 

 fore not considered as accurate as the others. If this is a second 

 order line its time wave lengrth is .708 (using Dershem's value 

 1.416) which compares well with the wave length .706^. The 

 fact that this line lies in the second order bromine absorption 

 band makes it difficult to observe. 



The line listed by Dershcm as 1.177'' lias been found to have a 

 wave len.irth 1.202^ A comparison of photogTaphs shows that 

 these two values are meant for the same line. Since Dershem's 

 value was found from observations on only one plate and mine 

 from obseiwations on four plates, which agree well together, 

 Dershem's value for this wave length is undoubtedly in error 

 much beyond his limit of experimental error. 



The line of wave length .792^ appears light on the plates in 

 contrast to the others which appear dark. There may be two 

 explanations for tbis. 



1. It may be the edge of an absorption band due to some 

 substance through which the rays pass before reaching the plate. 

 The rays pass through glass, air, and paper. The edge of the 

 absorption band would be the short wave length end of the 

 spectnun, but it would appear tliat the K-series wave lengths 

 of these substances are far too large for this region. The "J 

 radiations" found by Barkla and White^ for carbon, oxygen, 

 and aluminum have wave lengths far too short for this region. 

 This assumption is therefore not very probable. 



2. It may be the convergence Avave length of a series as yet 

 unknown. The end of a series would appear as a dark band 

 with a lighter region at the short wave length end. This sudden 

 change from black to light is probably what has been observed 

 here. This line is in the region of the bromine absorption band 

 and the general blackening of the plate makes it hard to obser\'e. 



The line of wave length 1.213'' makes a close doublet ^nth the 

 line 1.209' already found by Dershem. The close proximity of 

 these two lines, which were clearly resolved, sIioaa'S that the re- 

 solving power of the apparatus was at least 336 for that wave 

 length. 



Physical Laboratory 

 The State University. 



