256 ANDERSON— ECLIPSE EXPEDITION OF THE 



the axis of the concave mirror was incHned to the south and up- 

 wards sufficiently to allow the beam to clear the coelostat mounting, 

 the angle being a matter of about 3°. 



This arrangement which is unusually compact even for eclipse 

 work was found to work very satisfactorily indeed, and on account 

 of its compactness it was a simple matter to protect all the instru- 

 ments from the heat of the sun, as well as from vibration due to the 

 wind. 



The Littrow spectrograph consisted a 6-inch aperture, 18-foot 

 focus lens, and a plane grating having a ruled surface of 4 X 6 

 inches, I5,cx)0 lines per inch, one first prder being extremely bright. 

 The dispersion was about 3.4 A.U. per millimeter, which was 

 judged sufficient for accurate wave-length determinations. The 

 diameter of the moon's image formed by the projection mirror was 

 about three inches, and this image was placed just tangent to the 

 slit, the point of tangency being nearly coincident with the position 

 of second contact. During mid-totality, the whole spectrograph 

 was moved parallel to itself a distance of three inches so that during 

 the last half of totality the slit was tangent to the moon's limb at or 

 near the point of third contact. An auxiliary 90° prism and observ- 

 ing eyepiece enabled one of the observers to watch directly the 

 region of the spectrum including D3 for purposes of accurate guid- 

 ing. Two small right-angled prisms placed over the slit allowed on 

 iron arc comparison spectrum to be impressed on each plate simul- 

 taneously with the exposure on the sun. The exposures were six 

 in number, three made during the total phase and three before and 

 after, in order to get accurate wave-lengths extremely close to the 

 sun's limb. 



The corona rotation spectrograph was a three-prism Littrow 

 type, using a lens of forty-inch focus, the dispersion at A 5300 being 

 about six A.U. per millimeter. Only that portion of the spectrum 

 in the immediate vicinity of the green coronal line was photo- 

 graphed, the slit being parallel tO' the sun's equator. An iron arc 

 comparison spectrum was impressed immediately before totality so 

 as to permit an accurate determination of the wave-length of the 

 green line. 



The objective grating spectrograph used a six-inch Rowland 



