112 ON A TABLE OF STANDARD WAYE LENGTHS OF THE SPECTRAL LINES. 
Reducing these values by 1 part in 200,000, we make them agree with the 
uhsolutc vsihie o^ the standard as before agreed upon. Thus the column of stau- 
durds is obtained f()r use in the visible spectrum. 
For ordinary interpolation with the short and imperfect micrometers generally used, 
and ^Y0^king with a flat grating and a spectrum not normal, the standards would be too 
fur apart. But with such a long and perfect micrometer as I use, and working with 
the normal spectrum of a concave grating, they are entirely sufiicient. However, I 
hiive fdlud in the interval from 7030 to 7621 by some extra sub-standards at 7230. 
The micrometer for eye observations has a range of five inches, and the machine 
for measuring photographs of more than twenty inches, both with practically perfect 
screws made by my process. The eye observations are not an interpolation, in the 
ordinary sense, between the standards, but the whole series is continuous, the mi- 
crometer observations overlapping so that they join together to any length desired, 
liy measuring from the B line in one spectrum to the D line in the next, and 
I 
including the overlapping spectra, no further standards would be necessary, as 
all the linos of the spectrum would be determined at once, knowing the wave 
length of the D line. But I usually plotted the difference of the standards from 
the micrometer determination, usually amounting to less than one or two tenths 
of a division of Angstrom, and so corrected the whole series to the standards. 
Somctnnes two, or even three, overlapping spectra were measured at once. 
To make Table X., the followin.o: process was used. 
1st. From all the ob 
o 
my disposal, I determined a few mor 
around the main standards, and put them in the second column, marked St., so 
that 1 should have a greater number of points to draw my curve through. 
2d. I then put down a few obscFvations which were made by measuring over- 
lapping spectra. 
3d. Then the main eye observations were put down as follows : 
!> extending from 4071 to 7040, 2d spectrum, 14,43G grating. 
1 " " 4009 to 7035, « « u u 
^ " " 4859 to 7040, « " « « 
c " « 
C « « 
h « « 
i (( u 
Cf « « 
4ho9 to 6079 (fragmentary). 
6855 to 6909, 2d spectrum, 14,436 grating. 
5162 to 7201, « « i( « 
6742 to 7628, 1st spectrum, 14,436 grating 
m-^ I 11-. » ^ ^L .M^ ^^^ mW ^^ '^ . . 
ii u 
< " " 6499 
6065 to 7671, " ' a 
6855 to 7714, plane grating. 
5139 to 5296, 2d spectrum, 14,436 grating 
a 
« u 
i< a u 
E '< ^i 
6278 to 6322, « u « 
404S to 4824, « « u a 
