TUB TOTAL SOLAR ECLIPSE. 99 



The difficulties of solar study, in spite of comparative nearness and 

 intense brightness, are very great. It is not generally appreciated that 

 we are unable to study the body of the sun except by indirect methods. 

 The interior is invisible. The spherical body which we popularly 

 speak of as the sun is bounded by the opaque photosphere — a cloud 

 covering composed of condensed vapors of the metallic elements. The 

 photospheric veil, including the larger interruptions in it which we 

 call the sunspots; the brighter areas, closely connected with the photo- 

 sphere, called the faculae; the reversing layer, a few hundred miles in 

 thickness, immediately overlying the photosphere; the chromosphere, 

 a shell several thousand miles thick, associated with and overlying the 

 reversing layer; the prominences, apparently ejected from the chromo- 

 sphere; and the corona, extending outward from the sun in all direc- 

 tions to enormous distances; these superlatively interesting features of 

 the sun, constituting the only portions accessible for direct observation 

 by telescope and spectroscope, are an insignificant part of its mass. 

 They are literally the sun's outcasts. Our knowledge of the sun is 

 based almost entirely upon a study of these outcasts. We might hope 

 to reach safe conclusions as to the characteristics of a hermit nation 

 by making a careful study of its banished subjects, provided the ob- 

 served types correspond with types produced by our own civilization; 

 but if new types, new customs, new forms, presented themselves, and 

 were observable only at long range, our conclusions as to the charac- 

 teristics of the country from which they were expelled would come 

 slowly and uncertainly. It is a difficult matter to comprehend the 

 structure and condition of any one of the sun's outcasts; the chromo- 

 sphere, for example. To determine what the conditions within the 

 body of the sun must be in order to create and maintain such an out- 

 cast shell is far more difficult. 



The influence of eclipse observations upon solar and astrophysical 

 research has been most remarkable. The reversing layer, the chromo- 

 sphere, the prominences and the corona were in fact discovered at 

 eclipses. Many of our present every-day methods of studying them 

 are also eclipse products. The richness of eclipse results, considering 

 the remarkably short intervals available for observation, is unique in 

 science. To realize this, we need only recall that the durations of 

 observable total eclipses, clear and cloudy, have amounted altogether 

 to about one hour since the spectroscope was applied to the problem, 

 and about half an hour since photographic methods have prevailed. 



Eclipse problems relate not only to the properties of the less massive 

 portions of the sun — everything, apparently, outside of the photo- 

 spheric layer — but to the question of possible planets between the sun 

 and Mercury. It is well known that mathematical theory, based upon 

 Newton 's law of gravitation, has not yet fully accounted for the motion 



