MOUNT WILSON OBSERVATORY. 195 



The plate has been measured by Mr. Adams, Miss Ware, and Mr. 

 St. John. The results found by the use of the Hartmann formula, 

 corrected for the motion of the earth, are: 



Adams 5303 . 205 angstroms 



St. John 204 



Ware 203 



Mean 5303 . 204 Rowland System 



or 5303 . 022 International System 



As this value represents the wave-length at the east limb, it is 

 subject to an unknown correction due to rotation. If, however, it 

 be assumed that the rotation of the corona is of the same order as 

 the rotation of the higher chromosphere, namely, that on the east 

 limb it is approaching the earth with a velocity of 2 km. per second, 

 the correction is +0.035 A, making the Rowland and International 

 wave-lengths 5303.239 and 5303.057 A, respectively. The main 

 value of the present determination is that it establishes the wave- 

 length of the green coronal line in terms of iron-arc standards and 

 furnishes a basis for comparison with future similar observations. 



TOTAL SOLAR ECLIPSE OF JUNE 8. 



The plan of observations for the total solar eclipse of June 8 was 

 based up0n certain specific problems arising from otir experience in 

 solar work during recent years. Since most observers at previous 

 eclipses have u^ed apparatus of moderate power for spectroscopic 

 investigations, it seemed desirable in the present case to employ 

 instruments capable of yielding results of higher precision and more 

 nearly comparable with those obtained with the solar spectroscopes 

 in regular use. IVIoreover, essentially all the values for the wave- 

 lengths of the bright lines in the spectrum of the flash are relative in 

 character and provide no information regarding the possible displace- 

 ments of these lines with reference to the dark lines of the Fraunhofer 

 spectrum. Accordingly a sUt spectrograph was designed, having an 

 apparatus for producing a comparison spectrum of the iron arc which 

 could be used for reference purposes. 



When completed, the equipment for the expedition comprised the 

 following instruments : 



1. A photoheliograph with an aperture of 8 inches and a focal length of 30 feet for direct 



photography of the corona. 



2. A wide-angle camera with an aperture of 4 inches and a focal length of 18 inches for 



photographing extensions of the coronal streamers. 



3. A concave grating spectrograph of the objective type with a focus of 10§ feet for a study 



of the spectrum of the flash and the corona, particularly in the ultra-violet. 



4. A slit spectrograph of 18 feet focal length with a plane grating of 7 inches aperture for the 



determination of accurate wave-lengths of the hnes in a portion of the flash spec- 

 tnim. 

 6. A three-prism slit spectrograph of the auto-collimating type for an investigation of the 

 rotation of the solar corona and the wave-length of the green coronal line. 



6. A small camera with apparatus for studying the polarization of the corona. 



7. A camera provided with a silvered-quartz lens for photographing the corona in light 



transmitted by the silver band. 



8. A small refractor and polarizing eye-piece for visual observations. 



