180 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1930 



The end results of the radioactive changes are helium and lead which 

 are being formed continually from their ultimate sources, thorium 

 and uranium, through the more highl}^ radioactive elements in the 

 series of transformations. That these do not appear in the sun may 

 be due in part to their radioactive degeneration, and the ending of 

 the series of elements Avith uranium, at No. 92, the heaviest element, 

 may be due to the disappearance of possibly heavier elements through 

 their radioactivity, so that we have left the more stable ones which 

 have proA^ed the fittest to survive. 



ASTROPHYSICS 



The methods introduced by Huggins and Lockyer of interpreting 

 astronomical spectra through observations in the physical laboratory 

 led to the development of astrophysics, as a branch of physical 

 science. Confirmation of the soundness and fruitfulness of these 

 methods could be drawn from many sides. An outstanding illustra- 

 tion is the Avork of Hale and his co-workers, who have brought us to 

 the present view of the constitution of sun spots as great vortices 

 whose centers are magnetic fields which may in the larger spots reach 

 intensities of 4,000 gausses OA^er areas of many millions of square 

 miles. 



MAGNETIC FIELDS IN SUN SPOTS 



Many lines in the spectra of sun spots taken with sufficiently high 

 dispersion are doubled or tripled, depending upon the strength of 

 the magnetic field and the angle between the lines of force and the 

 path of the light. The components of the doublets were found by 

 Hale to be circularly polarized in opposite directions. In the lab- 

 oratory similar doubling with circular polarization of the compo- 

 nents is obserA^ed when the light source is placed between the poles 

 of a powerful magnet and the light emitted by a spectrum line is 

 viewed in a direction parallel to the lines of the magnetic field. 

 Viewed in any other direction, the line is a triplet, the middle com- 

 ponent plane, and the two side components ellipticall}^ polarized; 

 except Avhen normal to the field, when the two side components are 

 plane polarized at right angles to the plane of the central compo- 

 nent. The illustration (pi. 4, fig. 1) shoAvs the Fe line, A6,173, photo- 

 graphed as an absorption line in a sun spot and as an emission 

 line in the laborator}^ (the insert). The horizontal bands are due 

 to quarter-Avavc mica strips witli their optical axes at right angles 

 to each other in the alternate bands. These change the circular or 

 elliptical vibration of the side components into plane vibrations at 

 right angles to each other in the alternate bands. A Nicol prism in 

 the optical train cuts out the blue and transmits the red (side) com- 



