236 Scientific Proceedings, Royal Dublin Society. 



Kayser, the latter observer's numbers have been adopted. In 

 those cases in which they have not agreed, they have been 

 confirmed, or corrected, by re-measuring the lines with a micro- 

 scope and eye-piece micrometer, by Zeiss, the photographic plate 

 being mounted on a carriage actuated by a micrometer screw. 

 The lines were measured in reference to a standard scale formed 

 by the lines of a photograph of a reseau belonging to the 

 Observatory of Cambridge University. 



In the author's photographs of the platinum and chromium 

 spectra 443 lines have been measured between the two extreme 

 limits of wave-length 2229 '45 and 4560*21 in the former, and 

 1283 in the latter. Kayser gives between the same limits the 

 wave-lengths of 497 lines. Exner and Haschek 1 have given in 

 their list of the chromium lines, between the above limits of wave- 

 length, measurements of 2027 lines. 



The author again has to acknowledge his indebtedness to 

 Miss M. Hall for the very valuable assistance she has given in 

 the work of making the micrometer measurements, and of calcu- 

 lating from them the wave-lengths recorded in this communica- 

 tion. 



1 Kais. Akad. d. "Wissensch., Wien, Sitzungsb. Math.-Naturw. CI., Vol. cvi., 1897, 

 p. 1133. 



