154 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



scattered sky light was less than five-millionths that of the sun, 

 Lyot not only was able to photograph the brighter parts of the inner 

 corona hut by the use of a spectrograph he determined the wave 

 lengths of many coronal lines including three lines in the infrared not 

 yet observed in eclipses. 



THE LIGHT OF THE CORONA 



"Our knowledge regarding the distribution of light ^^^thin the corona 

 is in a very unsatisfactory state since the law of intensity has been 

 found by various observers to be inversely as the second- fourth, 

 sixth, seventh, eighth, and powers such as 2.3 or 2.4, of the distance 

 measured either from the center or from the edge of the sun. It is 

 of great importance that a well-devised form of apparatus be con- 

 structed for use at eclipses, and that photogra])hs be secured both in the 

 violet and visual regions on a carefully prepared plan at several future 

 eclipses. The eclipse photographs should have calibrated squares 

 impressed on them from a standard source, and if in addition photo- 

 graphs of the full moon were obtained, we should then be in a position 

 of having information additional to that acquired during the progress 

 of the eclipse. We need to know whether the distribution of light 

 within the corona follows the same law at every eclipse, or whether 

 this law varies according to the sunspot period, and we need to know 

 the law both in the blue and the j^ellow regions. Many coronas have 

 been observed through clouds or haze or with varying conditions of 

 transparency. The intensity of the silver deposit measured on the 

 photographs is a summation of two separate effects, one due to the 

 corona itself, the other to the diffuse light of the sky. Added to these, 

 there is in reaUty a third effect found close to the moon's limb, a halo 

 caused by reflection from the glass-side of the plate of the strong 

 illumination of the inner corona. Unfortunately, it has not been 

 possible to make proper allowances for these varying factors, with the 

 result that observations are affected by large systematic errors. 



In considering the measurements made for determining the total 

 light of the corona, there are similar evidences of large systematic 

 errors depending on transparency conditions at the time of the eclipse. 

 A few examples may l)e given to show the wide divergent results 

 obtained at the same eclipse by different methods and at different 

 eclipses by the same methods and instruments. Harvard Observa- 

 tory devised a photographic photometer which has been used since 

 the 1925 eclipse. Various types of illumination meters are also used 

 visually which can furnish a measure of the total illumination of 

 corona plus diffused sky light. Measures made with the i)hoto- 

 graphic photometers at the eclipse of 1925 and 1926 seemed to show 

 the corona 40 percent brighter in 192G than in the year previous. 

 Visual work with the illumination meter in 1925 gave 0.24 foot- 



