1012] on the Total Eclipse of the Sun, April 1911. 853 



secured illustrating intimate association between promineucu ami the 

 overlying coronal material, thus affording further evidence of their 

 close connexion. 



While, therefore, prominence activity is most likely responsible 

 for providing and raising the material from the body of the sun in 

 the various latitudes according to the different epochs of prominence 

 activity, what action is it that organizes and arranges the streamers 

 which extend sometimes five or six millions of miles into space ? 



The close association between the occurrences of terrestrial 

 magnetic storms and solar disturbances, and the results of the 

 researches which were described m this Institution in 1909, namely, 

 the discovery of solar vortices and the presence of powerful magnetic 

 fields which result from the revolution of the negatively charged 

 particles, termed corpuscles, in them — these suggest strongly a cause, 

 namely electromagnetic action, to explain the effect. 



So long, then, as the corona can only be observed during eclipses, 

 the study of its general form and its structure in close proximity to 

 prominences should be minutely recorded and discussed. 



About the chemistry of the corona nothing is known. The 

 spectroscope on many occasions has permitted observers to photograph 

 the many radiations that it emits, and while numerous determinations 

 of the wave-lengths of these radiations have been made, no terrestrial 

 equivalents have yet been discovered. Thus its composition is still 

 a mystery. 



For the occasion of last year's eclipse the main work intended to 

 be accomplished by the expedition of which I was in charge was a study 

 on a large scale of the spectra of the chromosphere and corona 

 together with the form of the corona. 



For the spectroscopic work, two large instruments giving large 

 dispersion were constructed, adjusted and taken out, and several 

 coronagraphs of different focal lengths, namely IC>. 7, 4 and 8 feet, 

 were utilized for recording the form of the corona. 



The large spectroscopes were of two kinds, one being of the " slit " 

 type employing a 10-foot concave grating, while the other was of the 

 " slitless " or prismatic camera type, in which the dispersion was 

 secured ])y four six-inch prisms of 45° angle. With the former it 

 was specially intended to secure photographs from which the wave- 

 lengths of the lines in the spectra of the chromosphere and corona 

 could be determined. 



This slit form of instrument is the most efficient for such measures, 

 for the spectrum consists of images of the fine slit and appears as a 

 series of sharp fine lines, the wave-lengths of which can be accurately 

 measured. 



Such a spectrum, however, tells one nothing about the form of 

 the object the radiations from which are l)eing studied, but simply 

 about the chemical composition of that portion of the image the light 

 of which passes through the slit. 



