KELATIVE WAVE-LENGTH OF LINES OF SOLAK SPECTRUM 515 



have now been made of nearly all the lines below & with a view of mak- 

 ing 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 II 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, however, used and the relative wave-length of the 

 standard lines, marked S in Table II, was obtained; however, some 

 other observations were also included. 



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

 lute 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 meas- 

 ured 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. 



This table is to be used in connection with my photographic map of 

 the normal spectrum to determine the error of the latter at any point. 

 The map was made by placing the photograph in contact with the scale, 

 which was the same for each order of spectrum, and enlarging the two 

 together. In this way the map has no local irregularities, although the 

 scale may be displaced slightly from its true position, and may be a little 

 too long or short, although as far as I have tested it, it seems to have 

 very little error of the latter sort. The scale was meant in all cases, 

 except the ultra violet, to apply to Peirce's absolute value and so the 

 correction is generally negative, as follows : 



Approximate correction to the photographic map of the normal spectrum to 

 reduce to latest absolute value. 



Strip 3200 to 3330 Correction -05 



" 3275 to 3530 " -05 



" 3475 to 3730 " -02 



" 3675 to 3930 <* -10 



" 3875 to 4130 " -16 



" 4075 to 4330.. " ...-04 



