330 ANNUAL KEPOKT SMITHSONIAN INSTITUTION, 1908. 



light from the red component should be transmitted and that of the violet com- 

 ponent cut off. When rotated 90° in azimuth, the Nicol should transmit the 

 violet component and cut off the red component. Complete extinction of either 

 component is hardly to be expected, because the light from the spot does not, in 

 general, come exactly along the lines of force, and the doublets may therefore 

 exhibit some traces of elliptical polarization. Moreover, the beam of sunlight 

 undei-goes two reflections on the silvered surfaces of the coelostat and second 

 mirrors of the tower telescope, where elliptical polarization must again be in- 

 troduced." By setting the rhomb at the proper angle, the latter effect, which 

 is not very large, can be almost wholly eliminated, but the former may play 

 some part, even when the spot is at the center of the sun. 



The light of the spot, after transmission through the rhomb and Nicol, comes 

 to a focus in the plane of the slit. While photographing the spot spectrum 

 the slit is covered except at its central part, where a portion corresponding in 

 length (from 1 to 2 millimeters) to the diameter of the umbra, receives the 

 light. During the exposure, which may continue from a few minutes to over 

 an hour, the image of the umbra is kept as nearly as possible central on the 

 slit, any irregularities in the motion of the driving clock being corrected by the 

 observer. As the exposure for the spot spectrum is from five to twenty times as 

 long as for the solar spectrum, it is evident that care must be taken to prevent 

 light from regions outside the spot from entering the slit. 



For a comparison spectrum sunlight is used, generally from a point in the 

 solar image a short distance away from the spot, where none of the character- 

 istic spot phenomena appear. During the exposure, that part of the slit which 

 previously received the light of the umbra is covered, and sunlight admitted on 

 either side. The light of the comparison spectrum passes through the rhomb 

 and Nicol, both of which occupy the same positions as in the case of the spot. 

 Care is taken to see that the grating is fully illuminated, both for the spot and 

 comparison specti'a, in all positions of the Nicol. 



CIRCULAR POLARIZATION ALONG THE LINKS OF FORCE. 



My first observations were made on June 24, in the second order of the 

 grating, but the results were not conclusive. On June 25 I obtained some good 

 photographs in the third order, of the region X 6000-6200, using Seed's process 

 plates, sensitized for the red by Wallace's three-dye formula.'' These clearly 

 showed a reversal of the relative intensities of the components of spot doublets 

 when the Nicol was turned through an angle of 90°. Moreover, many of the 

 widened lines were shifted in position by rotation of the Nicol, indicating that 

 light from the edges of these lines is circularly polarized in opposite directions. 

 The displacements of the widened lines appeai*ed to be precisely similar in 

 character to those detected by Zeeman in his first observations of radiation in 

 a magnetic field. 



A series of photographs, made with the Nicol set at various angles, soon 

 showed the two positions giving the maximum effect. At these positions the 

 weaker components of the strongest doublets are not always completely cut 

 off, but their intensities are greatly reduced. Sometimes hardly a trace of 

 the Weaker component remains, as may be seen in the case of the vanadium 

 doublet at \ 5940.87 (pi. 4). In this plate No. 5 shows the doublet in the 

 ordinary spot spectrum, photographed without the rhomb and Nicol. No. 4, 



^A study of the elliptical polarization of these mirrors has been made by 

 Doctor St. John. 



" Astrophysical Journal, Vol. XXVI, p. 299, 1907, 



