260 Prof. H. A. Rowland on the Relative Wave-lengths 



It is impossible for me to give at present all the data on 

 which my determinations rest ; but I have given in Table I. 

 many of the coincidences as observed with several gratings, 

 the number of single readings being given in the parentheses 

 over each set. 



Table II. gives the wave-lengths as interpolated by the 

 micrometer. It is scarcely possible that any error will be 

 found (except accidental errors) of more than "02 ; and, from 

 the agreement of the observations, I scarcely expect to make 

 any changes in the final table of more than 01, except in the 

 extremities of the spectrum, where it may amount to *03 in 

 the region of the A and H lines. The wave-lengths of weight 

 greater than 1 will probably be found more exact than this. 

 The lines can be identified on my new photograph of the 

 spectrum down to 5790. Below this there is little trouble in 

 finding the right ones. All maps of the spectrum, especially 

 above F, are so imperfect that it is almost impossible to 

 identify my lines upon them. The lines can only be properly 

 identified by a power sufficient to clearly divide b 3 and 6 4 . 

 !8ome of them are double, and most of these have been 

 marked ; but as the table has been made for my own use, I 

 have not been very careful to examine each line. This will, 

 however, be finally done. Micrometric measures have now 

 been made of nearly all the lines below 6, wdth a view of 

 making a map of this region. 



Table I. gives the coincidences of the different orders of 

 the spectra as observed with several concave gratings on both 

 sides of the normal, the numbers in the brackets indicating 

 the number of observations. The observations have been 

 reduced as nearly as possible to what I consider the true 

 wave-length, the small difference from the numbers given in 

 Table I. being the variation of the observations from the 

 mean value. The true way of reducing these observations 

 would be to form a linear equation for each series and reduce 

 by the method of least squares. A simpler way was, how- 

 ever, used, and the relative wave-lengths of the standard lines 

 (marked S in Table II.) were obtained ; however, some other 

 observations were also included. 



Table II. gives the wave-lengths reduced to Bell's value for 

 the absolute wave-length of the D line. These were obtained 

 by micrometric measurement from the standards, as described 

 before. The weights are given in the first column, and some 

 of the lines, which were measured double, have also been 

 marked. But the series has not yet been carefully examined 

 for doubles. The method is so much more accurate than by 

 means of angular measurement, that the latter has little or no 

 weight in comparison. 



