236 CARNEGIE INSTITUTION OF AVASHINGTON. 



the same angle as that used in the solar work, we can obtain pairs of 

 photographs which unite into true stereograms. It may be added that 

 we have confined ourselves to Helmholtz's limiting angle and have not 

 adopted the much smaller value given by Pulfrich. When used with 

 care, and with sufficient precaution against possible errors, these 

 stereograms may be useful in studying the relative levels of the flocculi. 

 It may be added that they give results in haraiony with conclusions 

 obtained by other means. 



A NEW MAP OF THE SUN-SPOT SPECTRUM. 



In 1907 we prepared and distributed to observers a map of the sun- 

 spot spectrum on a scale of 2 mm. to the angstrom. Some very fine 

 photographs of the spectrum of a large spot, recently made by Mr. 

 Ellerman in the second order of the 75-foot spectrograph, have rendered 

 possible the preparation of a map on a scale of 1 cm. to the angstrom, 

 which is sufficient to show the chief phenomena of the Zeeman effect, 

 including some interestmg anomalies.^ The polarization phenomena 

 are brought out to special advantage through the use of a compound 

 quarter-wave plate, mounted in conjunction with a Nicol prism just 

 above the slit of the spectrograph. At present the map covers only 

 the region from X6000 to X6450, but it will be extended over a greater 

 range as soon as sufficiently perfect photographs of other regions can 

 be obtained. 



The definitive reduction of photographs of sun-spot spectra, which 

 involves the measurement of the components of triplets and quad- 

 ruplets, is seriously complicated by the physiological errors which 

 enter when one attempts to bisect closely adjacent lines. As the 

 discussion of the results demands an elimination of such errors, the 

 final studies have been further delayed until they can be evaluated 

 with the Koch machine or otherwise. Meanwhile the studies of Mr. 

 St. John (p. 240) have shown how seriously such errors may vitiate 

 conclusions wliich ignore their existence. 



THE ZEEMAN EFFECT IN SPOTS NEAR THE SUN's LIMB. 



When a spot is observed close to the sun's limb, the lines of force 

 of its magnetic field vary greatly in inclination at points but slightly 

 separated from one another. At the center of the spot, where they are 

 apparently radial, they He nearly at right angles to the hne of sight. 

 In this case the polarization phenomena should be those observed in 

 the laboratory across the field of a magnet, i. e., the three components of 

 a triplet should be plane-polarized, permitting the outer components or 

 the central one to be cut out at will with a Nicol prism. On the inner 

 edge of the penumbra the lines of force are directed toward the 



'Behavior of the p-components of triplets as though their edges were circularly polarized in 

 opposite directions; presence of triplets which indicate stronger fields in the penumbra than in 

 the umbra; transmission of both n-components of triplets at the center of spots where the com- 

 pound quarter-wave plate and Nicol completely cut off one component in the penumbra, etc. 



