368 
INTERNATIONAL ASTRONOMICAL UNION Proc. N. A. S. 
line (such as the bright yellow Neon line) over one from solid Cadmium 
as a source of light combine to urge a reconsideration even of the primary 
standard. 
Since a committee, consisting of Messrs. Ames, Anderson, Michelson 
and Stratton, has been appointed by the National Research Council to 
consider the practicability of adopting a wave-length of light as the 
fundamental standard of length, it is recommended that the question 
of a primary standard of wave-length be held in abeyance — that is, not 
reopened — until the above-mentioned committee makes its report. 
In case the International Bureau of Weights and Measures adopts a 
certain wave-length of light as their standard of length, it would appear 
to follow, as a matter of course, that this wave-length should be adopted 
also by the Astronomical Union, and that the numerical value attached 
to it should be that assigned by the International Bureau of Weights and 
Measures. 
As to the possible use of a Neon line, or the sharp component of the 
green Mercury line, or of a sharp line in the absorption spectrum of Iodine, 
for the fundamental unit of length, instead of the red Cadmium line, 
your committee feels that this is a matter belonging entirely to the Com- 
mittee of the International Bureau of Weights and Measures. 
3. Secondary standards. — A series of secondary standards is now being 
determined at the Bureau of Standards and at Mt. Wilson. The Committee 
recommends that the Astronomical Union encourage a third series of inde- 
pendent measures on these lines. 
It is recommended, therefore, that the International Astronomical Union 
arrange, if possible, with European spectroscopists for a determination of 
these secondary standards in Iron, Neon, and other substances which can 
be worklfd under definite and easily reproducible conditions. 
The dearth of Secondary Standards in the yellow-green region and in 
the infra-red appears to your committee an unsolved problem. Its solu- 
tion will be the discovery of a workable and easily duplicated source of light 
which will furnish properly distributed lines of sufficient sharpness and in- 
tensity. 
4. Tertiary standards. — Here the greatest need appears to be for lines 
to fill gaps in the ultra-violet, the yellow-green, and infra-red of Iron. 
With reference to this need your committee has no recommendation to 
make except to seek further information as to how the need may be met. 
For the measurement of tertiary standards independent determinations 
by the interferometer and by the grating are urgently recommended, on 
the ground that the errors of these two instruments are largely comple- 
mentary. The importance of this work is emphasized by the fact that 
it will be scarcely practicable to print a new "Table of Stronger Lines" 
for the identification of impurities and similar purposes until the tertiary 
standards are more complete than at present. 
