100 
ox A TABLE OF STANDARD WAVE 
OF THE SPECTRAL LINES. 
two glass and two speculum metal gratings, ruled on two dividing engines with 
u 
four varieties of spacing, three of which were incommensurable or nearly so, with 
two spectrometers of entirely different form, with a variety of standard bars compared 
in tills country and in Europe, and w^ith a special comparator made for the measure 
very well with the next best determination, that 
of ^ra tin LI'S. 
Ek 
result acrrees 
of Mr. C. S. Peirce of the U. S. Coast Survey. His final result agrees within 1 
60,000 with hi) 
of Pierce. Mii 11 
preliiuinary value.* This 
recent value, combined with those 
d Kempf, Kurlbaum and Angstrom, I have adopted to red 
my final results to, although the calculations are made according to Bell's preliminary 
value. See Appendix A. 
But it rests v 
absolute standard 
tific men at large to adopt some absolute standard 
The 
of 
so ira 
as the 
d possibly th 
average of Angstrom might be adopted. But for myself I do not believe in 
continuing an error of 
sort 
definitely. All the results obtained before the 
concave grating came into use were so imperfect, that they must be replaced by 
very soon. 
With 
good concave 
obtain the wave lengths of the elements 
known. 
g 
one man in a fe 
J 
th far greater 
acy 
ears could 
than now 
As an aid to this work, I have constructed the table of wave lengths given in 
paper, which ha\' 
already been adopted by the British Assoc 
nd by 
of Germany and other countries, and 
ely hope 
will 
aid in the work of making the wave length of a spectrum line a definite quantity 
within a few hundredths of a division of An^^strom. 

Absolute Wave Length of D. 
The following is an estimate of the absolute 
best det 
in which 
of coirectii 
affects the 
the 
b 
Fir^t, I shall 
of the rrratin 
w^ave length of 
the D line from 
the portion of Dr. Bell's paper t 
is founded on th 
space 
tal 
Tl 
me 
thod 
focal length, and 
principle that a linear error in the spaces 
not the 
tions D 
and thus throw the light far to 
Bell has used seem to nie v 
o 
d th 
small 
po 
w 
hich have 
appears to me to be twice too great. 
one 
'IT 
de, should be 
ted. The correc- 
oper, except to grating III 
which 
I find the folio win"; 
« 
American Journal of Science, 1887. 
t American Journal of Science, 1888 
