2 so POPULAR SCIENCE MONTHLY. 



would give witness to more rapid changes. From year to year large 

 changes in the form of the corona occur and these appear to be asso- 

 ciated with the sun-spot period. Tliis is a natural inference, especially 

 since the solar prominences are thus associated. This is well shown 

 by a comparison of the form of the corona in 1889 and 1900, wliich 

 occurred near the sun-spot minimum, with the form in 1893, which 

 was near sun-spot maximum. These are given in Figures 6, 7 and 8. 

 The equatorial streamers and the divergent polar streamers are much 

 more pronounced at the time of sun-spot minimum. At maximimi the 

 corona is more nearly circular. The polar streamers are beautifully 

 shown in Figure 9, a photograph made by the eclipse party, which 

 was under the direction of Secretary Langley, of the Smithsonian In- 

 stitution. The true nature of the corona and the complex changes 

 which it undergoes are unknown. The spectroscope is the magician's 

 wand which science generally uses to reveal the constitution of un- 

 known objects, but in this case the revelation is only partial. In 1869 

 Professor Young found the spectrum to be characterized by a bright 

 line in the green, which he identified as Kirchhoff's line 1474. The 

 unknown substance which produces this line has been given the name 

 'coronium.' There are also other less conspicuous bright lines. When 

 the name 'helium' was assigned to the origin of certain lines in the 

 solar spectrum, no such terrestrial substance was known. Later it was 

 found by Eamsay. A similar issue for coronium would be very accept- 

 able. The corona also yields a faint continuous spectrum, in which 

 Janssen and others have reported certain dark lines of the solar spec- 

 trum. This signifies, that in addition to luminous gases, giving a 

 spectrum of bright lines, the corona contains some substance, like a 

 cloud, which is capable of reflecting ordinary sunlight. A part of the 

 light appears to be polarized. It is thought by some observers that 

 there is also a bright continuous spectrum free from dark lines. If 

 true, this would imply a three-fold origin to the coronal light. For 

 the explanation of the corona we have the diffraction theory of Has- 

 tings, the mechanical theory of Schaeberle, the magnetic theory of 

 Bigelow, and others. The complete solution of the problem is of the 

 greatest difficulty and of the greatest importance. At the eclipse of 

 1900 some experiments with that remarkable instrument, the bolometer, 

 appear to throw new light on this subject. These experiments were 

 made by Secretary Langley's chief assistant, Mr. C. I. Abbott, who 

 reached the following conclusions : 



These observations indicate not only that the coronal radiation is very 

 slight, but that the apparent temperature of the inner corona is below 20° C. 

 For it will be noticed that the bolometer lost heat hy radiation to the corona, 

 as evidenced by a negative deflection. Hence, when we consider its visual 

 photometric brightness at the point where the bolometric measures were taken, 



