ON A TABLE OF STANDARD WAVE LEXGTnS OF TTTE SPFOTTIAL I.JM s. [Q 
o 
Placing the negatives so obtained on a dividing engine \\\{h a microscope of 
very low power and .a tightly stretched cros.s liair, the coinridciicc of ibc two 
spectra can be measured. Owing to the large scnle of the photogrnidis, ■— about 
;trom, — an ordinary dividing enirine Iiavin;: errors not crontor tlian 
tliat of Ang 
rijo inch can be used, but the negatives should be gone over at least twice, reversing 
them end for end. Two screws were used in the enicine and finally another 
plete machine was constructed, giving wave Icngtlis direct with nidy a pUgl 
correction. For determining the wave length of metallic lines, the same pn«".-«s 
can be used with wonderful accuracy. 
The results are given in the columns marked PI. with ihe number of the plate*. 
The accuracy is very remarkable, and I think the figures cstabli^;h the a«»s« rti(tn 
that the coincidence of solar and metallic lines can 1)e determined with a pmbnhle 
error of one part in 500,000 by only one observation. 
This process not only gave me measures of the ultra violet, but also now 
observations of the visible spectrum. So far in my work on these coinoiilcnrci, 
I have only used erythrosin j^lates going a little below D; but cyanine plates miglit 
be used to B, or even in the ultra red, as Trowbridge hiis recently shown. One 
plate, No. 20, however, connects wave lengths 04:00 and 3200. 
Thus I have constructed a table of about one thou^nnd lines, more or la», 
which are intertwined with each other in an immense number of ways. They 
have been tested in every way I can think of during eight or nine yen 
stood all the tests ; and I think I can p 
that the results of the relative measures 
Ilh confidi 
be altered very muoh. 1 bel 
that no systematic err<^ in the relative wave lengths of more than about ± .01 
any 
pt ill the red end as we approacli A. Possibly ± .03 
less, mis^ht cover that ret^ion. 
-3..^ V.V. , V... w...„v. .^_ 
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The relative measures having thus been obtained, we have means m the concave 
of obtainino- the wave leuLrths of the lines of metals to a degree of nccurnoy 
«j ..^ V,^......W._ V..W W...W *-.-^ 
hitherto unknown, and thus of solvinn: the great problem of the ma 
o ""^ o 
distribution of these lines. 
But for the comparison of spectra, as measured by diiTerent observers, some 
absolute scale is needed. Hitherto Angstrom has been used. But it is now very 
well known that his standard measure was wrong. As his relative mcacures are 
also very wroncr, I have concluded that the time lias come to change not or 
ite also. To this end Dr. Louis Bell worked 
D 
the relative measures, but tlie'absol 
% 
my laboratory for several years with the best apparatus of modern science 
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