230 CARNEGIE INSTITUTION OF WASHINGTON. 



than heretofore. Within the hmits of observational error, stable lines 

 have been found to have the same wave-lengths in all the different 

 forms of arcs which we have tested. 



For the unstable lines the need of a definition of the fundamental 

 wave-length has become evident. Provisionally it is defined as the wave- 

 length free from displacements due to conditions in the arc itself, 

 which are associated with proximity to the poles but extended in range 

 by increase of current. Evidence that a fundamental state may be 

 attained is furnished by three lines of investigation: change of arc 

 length, of current strength, and of constituents of the negative pole. 

 When no further variation in wave-length accompanies a change of 

 any one of the variables, the other two being constant, it is assumed 

 that the fundamental condition has been reached. 



A study of the widths of iron lines in various sources has shown that 

 at atmospheric pressure the Pfund arc with negative pole of carbon 

 yields the narrowest lines, especially when operated at a length of 

 10 to 15 mm., with a current of 4 or 5 amperes. A close rival is found 

 in the center of the 5-ampere 12 mm. Pfund arc, which is the source 

 reconmiended by Mr. St. John and Mr. Babcock for the production 

 of secondary standards of wave-length. The center of the international 

 arc is far inferior in this regard, and many lines in it are widened to such 

 an extent as to be wholly unsuited to interference measurements. 



As a source of reference standards for the comparison of the relative 

 intensities of spectral lines, the center of the 5-ampere 12 mm. Pfund 

 arc is found to be superior to the other forms mentioned above. This 

 is due to the fact that the unstable lines are well developed in it, but 

 still have moderate intensity gradients from the center of the arc to 

 the poles. 



SECONDARY STANDARDS OF WAVE-LENGTH. 



For reasons set forth above, namely, the need of fundamental wave- 

 lengths for unstable lines and increased reproducibility for all classes 

 of lines, it became necessary to determine secondary standards in the 

 center of the 5-ampere 12 mm. Pfund arc. This has required a repeti- 

 tion of part of the work noted in the last annual report, although the 

 majority of the measurements of early plates can still be used. Deter- 

 mination of wave-lengths by the interference method were discontinued 

 until the completion of the study of sources, but the investigation is 

 now well advanced under improved obser\dng conditions; 37 new 

 photographs, all from a source free from pole effect, are now being 

 reduced. On these plates 506 iron lines between the limits X 3370 and 

 X5085 have been determined in terms of selected lines from the published 

 Ust of secondary standards. The increased sharpness in the iron lines 

 yielded by the source adopted for our work makes possible the use of 

 higher orders of interference than have hitherto been employed, and 

 this involves a considerable increase in the amount of work, not only 



