252 



PROCEEDIiNGS OF THE AMERICAN ACADEMY 



the residuals is given in the last column. The residuals of the H line, 

 being all rendered zero by the method of reduction, are not included 

 in the last column, although retained in the preceding columns. 



The average deviation of all the measures, including the faint, as 

 well as the more marked lines, is only 1.7, or less than .\ of the interval 

 between the sodium lines. Less accuracy was attained in measuring 

 the lines near the ends of the spectrum, owing to the faintness of the 

 photographic effect. In many cases the last lines were marked as 

 " very doubtful." This error was increased for the lines of greater 

 wave-length by the fact that a given distance, as one division of the 

 screw, represents a change in wave-length of double the corresponding 

 amount at the other end of the spectrum. Without the correction in- 

 dicated by this table, the average deviation of the individual results 

 would be 2.7. Table VI. gives the cori'ection indicated in each por- 

 tion of the spectrum derived from Table V. The first column gives 

 the limitinif wave-lenifths within which the corrections in the second 

 column should be applied. The sign of the corrections is such that 

 they are to be added to the results derived from the formula. 



TABLE VI. 



Measurements of these spectra may then be converted into wave- 

 lengths by ap[)lying to the readings of the curve described above the 

 corrections of Table VI. Thus the measurement 2.G18 gives by the 

 curve 38U2, but since this falls between 3758 and 3952, we must 

 apply the correction -j-l. The wave-length corresponding to 2.G18 

 will then l)e 38(>3. When the same lines are observed in several 

 spectra, the correction may be applied to their mean, instead of to the 

 separate i-eadings. 



Plates 21, 23, and 24 were taken with the 12-inch refractor, and 

 accordingly the spectra are narrow in the violet and wide in the blue 

 portions. For Plates G3, 65, 73, and 78 the 11-inch telescope was 



