110 ON" A TABLE OF STAXDARD WAVE LENGTHS OF THE SPECTRAL LINES. 
wonlrl be even ±.0000001 of the whole. This would detect a motion in the line 
of sight of ±110 feet per second! 
From the tests I have made on my standards, I am led to believe that down to 
wave length 7000, a correction not exceeding ±.01 division of Angstrom (1 part in 
500,000), properly distributed, would reduce every part to perfect relative accuracy. 
To ascend to the next degree of accuracy would need many small corrections 
which would scarcely pay. It is reasonable to assume that a higher degree of 
accuracy will not be needed for twenty-five years, as the present degree is sufficient 
to distinguish the lines of the different elements from one another in all cases that 
I have yet tried. 
Details of Woek. 
To red 
O"'^" IV, 
obser\' 
extending a short distance either side of the standard region are necessary. Thus 
the mean of 4215 and 4222 can be taken as the standard, and, if only one is observed, 
it can be reduced to the standard by a correction +3.358 or -3.358. But it is 
not necessary to take the mean of the lines as a standard, as any one of them may 
be so taken, or even any other point where there is no line, as the point is on 
to be used in the mathematical 
and finally disappears alto 
Table IT. gives results of this nature. The letters at the top of each series, 
^. U^ h, h etc., are the arbitrary names of the standards. The first columns refer 
to the series of observations, - Co." being observations made at the time of measurii 
cidences 
it 
' ^^^^^ ^' 1^' ^tc., refer to photographic plates; C, B, etc., refer 
to the series as given in the final table, although they may differ very slightly 
rom the latter, as the final table contains slight corrections. Fi-ures in parentheses 
are the number of readings. The photographs were usually measured from two 
to SIX times. • 
Table III. gives the first series of observations made in 1884 with a 21| foot 
concave, 14,436 lines to the inch. The numbers taken for the standards are only 
piennmary,and agree as nearly as practicable with my Table of Preliminary Stan- 
auis As only diferences are finally used, they are sufficiently near. The fractions 
give the order of the spectra observed 
Thus, the first ob 
Correction to standard _ .626 
on h and t is worked up as follows 
4G91.590 ' 7027.778 
+ 2.785 
4G90.964 7030.563 
