458 
ASTRONOMY: C. E. ST. JOHN 
ON THE SUGGESTED MUTUAL REPULSION OF FRAUNHOFER 
LINES 
By Charles E. St. John 
MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON 
Read before the Academy, April 18. 1916. Received, July 7.1916 
The correct interpretation of the results given by the spectroscope 
is so fundamental to progress in present day astronomy that every 
suggestion offering new points of view and avenues of approach to the 
problems furnished by the powerful instruments in commission deserves 
consideration. The spectrographs now used upon the sun yield spectra 
of such a scale and dispersion that minute changes in spectrum lines 
can be studied that were beyond the reach of the older instruments. 
Changes in the relative position of spectrum lines are the phenomena 
most often under consideration. All astrophysicists recognize two 
conditions that produce displacements of the Fraunhofer lines, motion 
in the line of sight and differences in pressure, both capable of precise 
determination. Two others have been suggested, the gravitational 
effect of Einstein^ and the anomalous dispersion hypothesis of Julius. ^ 
The former influence would displace all lines to a calculable amount and 
would, if found, add no serious difficulties to the solar problem. The 
effects of anomalous dispersion, however, would introduce an indetermin- 
ate factor into the spectroscopic problem. It becomes then a matter of 
prime importance to determine whether the relative positions of the 
Fraunhofer lines are measurably affected by this cause. 
An accepted deduction from the theory is a mutual influence between 
neighboring lines, a quasi repulsion increasing with the proximity of 
the adjacent lines. The violet and red components of a close pair 
must then be displaced to the violet and red, respectively, in comparison 
with the displacement of the isolated lines of the same class. Dis- 
placements between the solar and arc lines give a direct and definite 
means of testing the question. 
Iron, from its abundance of lines and the searching investigations 
given to them, furnishes the most reliable data. The wave lengths of the 
iron lines belonging to groups a, h and c4 of the Mount Wilson classi- 
fication are independent of arc conditions.^ These lines therefore are 
capable of giving results of the highest possible precision. The mean 
sun-arc displacement for 213 lines of these groups is -f 0.0039 A. Those 
who assign an important role to anomalous dispersion in the solar at- 
mosphere assume that solar lines within 0.5 A from each other are sub- 
ject to mutual repulsion. The mean displacement for 30 lines with 
