258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
questions were discussed and plans for future work outlined. The 
work of the union is carried on by several committees, whichy report 
at the triennial meetings. 
Probably the most important action taken was the adoption of a 
new system of wave lengths of light. The system in use for the 
last 20 years was introduced by Rowland, the values being obtained 
from measurements of spectra made with concave gratings. This 
system was far in advance of previous ones and was for a long time 
considered practically perfect. More recent investigations have 
shown, however, that not only were his absolute values in error, 
every wave length being too great by about 1 part in 30,000, which 
is not a matter of much moment, but that also—a much more serious 
question—there were relative errors of the order of about 1 part in 
100,000 among the different lines. These errors, due to unknown 
defects in the gratings, were only discovered when new measure- 
ments were made by a different method, that of interference. The 
new primary standard was first determined by Michelson in 1892 by 
actually counting the number of waves of the red line in the spec- 
trum of cadmium in a known fractional part of the standard meter. 
He found that there were 1,553,163.5 waves in a meter, equivalent 
to a wave length 0. 00064384722 mm., or, as it is asuelig written, 
6438.4722A. This value has more recently been confirmed by Fabry 
and Perot and is accepted as the primary standard of the new sys- 
tem. Secondary standards are composed of the wave lengths of 
50 lines, in the are spectrum of iron between 24282 and 46495, which 
have been independently measured by interference methods by three 
observers—Fabry and Buisson at Marseille, Eversheim at Bonn, and 
Pfund at Baltimore. The accordance of these measures is so good 
that the range is generally less than one part in a million, and the 
mean of the three is certainly correct, considerably within that 
margin of error. From these secondary standards tertiary standards 
are to be obtained by interpolation from grating spectra, and after 
these tertiary standards have been obtained new measures of the 
wave lengths of ail lines in solar and terrestrial spectra will be 
required. 
The importance of this work in silete and stellar investigations 
can not be overestimated, as many important results depend on the 
accuracy of wave-length values, and incorrect values may lead to 
erroneous conclusions. This is an instance of what I previously 
said of the necessary interrelation of astronomy and physics and 
the impossibility of successfully attacking modern astronomical prob- 
lems without the aid of the allied sciences. 
One of the important conclusions reached by the committee on 
sun spots was the practically unchanging character of sun-spot 
spectra. To this may be added the fact, conclusively proved by 
