THE EXPEDITION OF THE MOUNT WILSON OBSERVA- 

 TORY TO THE SOLAR ECLIPSE OF JUNE 8, 1918. 



By J. A. ANDERSON, Ph.D. 

 (Read April 25, 1919.) 



The apparatus of the Mount Wilson Observatory expedition at 

 Green River, Wyoming, consisted of : 



1. Littrow Plane Grating Spectrograph. Observers: Adams, St, 



John and Brackett. 



2. Corona Rotation Prism Spectrograph. Observers : Adams and 



Ellerman. 



3. Objective Grating Spectrograph. Observers: Anderson and 



Babcock. 



4. Objective Prism Polarizing Spectrographic Camera: Anderson 



and Babcock. 



5. Eight inch thirty-foot Photoheliograph Camera : ]\Ir. Ellerman. 



6. Small Silvered Quartz Lens Camera : Miss Margaret Hale. 



Light was supplied to all of these except (3) and (6) from the 

 thirty-inch ccelostat mirror of the Snow telescope. (3) was fed 

 from a small speculum mirror mounted on the coelostat axis, while 

 (6) was simply pointed directly at the sun, and the image allowed 

 to drift. 



The general arrangement is shown in the plan sketch (Slide i). 

 The beam of light from the coelostat mirror is directed horizontally 

 westward ; the lower central portion falls on the eight-inch photo- 

 heliograph lens, the portion immediately north of this serving to fill 

 the two-inch lens of the polarizing spectrograph camera. The 

 upper central portion falls on the six-inch image-forming lens of 

 the corona rotation spectrograph, while the considerable portion left 

 of the south half of the beam sufficiently fills the concave mirror 

 which forms the image used by the Littrow Plane Grating Spectro- 

 graph. This spectrograph was located east of the coelostat, and 



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