PHYSICS: ST. JOHN AND BABCOCK 
505 
and 51 cm. long, the ends of which were covered with w ndows of clear 
rock salt. This tube was evacuated with an oil pump and then filled 
with air which had been passed through phosphorous pentoxide. It 
therefore contained carbon dioxide which causes a small amount of 
absorption. 
The transmission was determined by noting a series of galvanometer 
deflections caused by black body radiation (800° C.) which was passed 
through the evacuated tube and focused upon a linear thermopile of 
bismuth-silver. Immediately thereafter a stopcock was opened and 
either dried or undried air was permitted to enter, under atmospheric 
pressure. 
Using air containing 9.95 grams of water per cubic meter the absorp- 
tion amounted to 0.9 %. 
Using dry air, the average value of the absorption (3 series of measure- 
ments) was 0.09%; which is the magnitude of the errors of observation. 
In view of the fact that in the measurements of the radiation constant, 
the column of (dry) air was less than 50 cm. if any correction was to be 
applied it could hardly be greater than 0.1%. 
, Millikan's calculations of o-, on the basis of recent determinations of 
Planck's element of action, from photoelectric measurements, and 
from his redetermination of the electron, is o- = 5.72 X 10"^^ =t= 0.034 
watt cm. ~2 (jeg. -4^ which is exactly the result I obtained by direct 
measurement. The only misgiving is that this coincidence is accidental. 
In conclusion it may therefore be stated that further experiments are 
in progress in which the whole radiometric apparatus is in a vacuum, 
the radiator being an enclosure surrounded by molten metal. 
iCoblentz, W. W., Washington, Bui. Btir. Standards, 12, 1916, (533). 
2Millikan, R. A., these Proceedings, 3, 1916, (231). 
^Coblentz, W. W., Washington, Bull. Bur. Standards, 9, 1913, (283). 
THE DEVELOPMENT OF A SOURCE FOR STANDARD WAVE- 
LENGTHS AND THE IMPORTANCE OF THEIR 
FUNDAMENTAL VALUES 
By Charles E. St. John and Harold D. Babcock 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Communicated by G. E. Hale, June 27, 1917 
The determination of wave-lengths in International Units forms an 
important part of the working program of a large number of laboratories. 
The primary International Unit is the absolute wave-length of the red 
cadmium line.^ The secondaries are referred individually to the primary 
