MOUNT WILSON OBSERVATORY. 257 



concave grating of twenty-one-foot radius of curvature, and was of 

 exactly the same type as has been used at various ecHpse observa- 

 tions before. One improvement in the method of observation needs, 

 hov^ever, to be noted: Ordinarily the spectrograph is adjusted with 

 the axis of the incident beam pointing to the center of the sun. For 

 a station near the center line of the eclipse track the direction of the 

 incident light at second and third contacts, make angles of ±15' 

 with this direction, which alters the focus of a grating such as was 

 employed here by a quantity of the order of one millimeter. In the 

 present case the speculum mirror being adjusted to the center of 

 the sun before totality was rotated eastward through an angle of 

 seven and one half minutes for the exposures at second contact ; 

 then back to the original position for the exposure at mid-totality ; 

 then westward seven and one half minutes for the remaining expo- 

 sures at third contact. 



The objective prism polarizing spectrograph consisted of an 

 ordinary 4X5 view camera having a two-inch aperture, eight-inch 

 focus Planar lens, in front of which was placed in order a direct 

 vision prism and a Rochon double image prism. Two objective 

 prism spectra are thus obtained, the planes of polarization being at 

 right angles to each other. 



The silvered quartz lens camera was used simply to obtain data 

 in regard to exposure time and relative aperture required for use at 

 future eclipses. As is well known a silver film opaque to visible 

 radiation is quite transparent from about A 3200 to A 3100. Used 

 on a quartz lens it is hence possible to obtain fairly monochromati*., 

 images in this part of the ultra-violet spectrum. In the present case 

 the aperture was about F/12, the exposure time through clouds with 

 an estimated transparency of 10 per cent., about ninety seconds, 

 and yet a great deal of coronal structure is well shown on the plate 

 obtained. Hence it should be easy to obtain good photographs in 

 this way when conditions are right. 



On the day of the eclipse a large cloud obscured the sun from a 

 few minutes after first contact until a few minutes after third con- 

 tact. During totality the sun was visible through an irregular thin- 

 ner portion of the cloud, but it is doubtful if the intensity of the 



PROG. AMER. PHIL. SOC, VOL. LVIII, Q, JULY 30, I919. 



