358 Palmer— Wave Length of the D, Heliwm Line. 
The D, line appeared only when the sun’s image was tangent 
to the slit and then as a bright but very short line in the center 
of the field of view vertically considered. Its definition and 
intensity were found to vary greatly from day to day, and for 
different points on the sun’s limb. In general, when a solar 
prominence lay across the slit the line was very broad and 
intense, but the definition of its edges was poor, thus render- 
ing it impossible to set the cross hairs on it with accuracy. 
The best combination of intensity and definition was obtained 
by avoiding prominences and working only on very clear days. 
The observations were made by the ordinary micrometric 
method, the D, line being compared with the best solar 
standard lines in the field of view. The wave lengths of these 
standard lines, as taken from Prof. Rowland’s ‘“ Table of Stan- 
dard Wave Lengths,”* were 
eee 2a 5916°475 Bess. Sus ee —5862°580 
We). 22 22 2 =. 914384 Pe <a ee 5859°810 
Be. eke ee 5905°895 ba. Ste eee 5853°903 
Na eee 5896°154 Average value . 5887°028 
Seventeen series of measurements were made, in each of 
which equal numbers of observations were taken on diametri- 
cally opposite points of the sun’s limb in order to eliminate 
the effect of rotation. 
The wave length of D, was calculated from each of these 
series by Prof. Rowland’s method of interpolation, on the 
assumption that, for the space used, the spectrum was essen- 
tially normal. The average of the seventeen values thus found 
gives 
5875°939 + °006 
for the wave length of the D, line, the probable error being 
calculated from the deviations of the several values from the 
mean in the usual manner. 
To test the accuracy of the observations and method of eal- 
culation, the wave length of the mean line was computed from 
the observations and found to be 5887-027, a value which dif- 
fers only by -001 from the average of the wave lengths of the 
standard lines used, 5887-028. : 
I am indebted to Prof. H. A. Rowland and Dr. J. S. Ames 
for permission to use apparatus and for suggestions, and to Mr. 
W.S. Day for aid in making the observations. 
Wilson Physical Laboratory, Brown University, Oct. 17, 1895. 
* Astr. and Astro.-Phys., vol. xii, p. 321, and Phil. Mag., V, vol. xxxvi, p. 49. 
