340 SMITHSONIAN MISCELLANEOUS COLLECTIONS \0L. 52 



If this solar image (obtained with reduced aperture of the concave 

 mirror) is permitted to fall for less than the thousandth part of a 

 second upon a photographic plate, a picture of the Sun will result. 

 Such pictures are made every clear day, in the early morning or 

 late afternoon, when the atmospheric conditions are at their best. 

 They show the Sun as it appears to the eye in visual observations. 

 The principal solar phenomena visible on such photographs are the 

 Sun-spots, several of which appear in Plate xxviii. These spots, 

 when observed imder the best conditions, are found to have an ex- 

 tremely intricate structure, which changes from hour to hour, and 

 sometimes from minute to minute, under the observer's eye. Indi- 

 vidual spots sometimes exceed 90,000 miles in length, but their area 

 is very small as compared with that of the entire solar disk. Thus 

 the great group of February, 1892, had a length of 166,000 miles and 

 a breadth of 65,000 miles. Its area was eighteen times as great as 

 that of the Earth, but only 0.15 of one per cent of the solar surface.^ 



Photographic Investigations oe Sun-spot Spectra 



In spite of the fact that Sun-spots have been under observation 

 for nearly three hundred years, little is known as to their true na- 

 ture. Various theories to account for them have been brought for- 

 ward, but the complexity of the phenomena and the lack of suffi- 

 cient observational data have stood in the way of accurate knowledge. 

 It is not certainly known, for example, whether Sun-spots are to be 

 regarded as elevated regions or as depressions below the general 

 level of the solar surface. Even the cause of their darkness has re- 

 mained uncertain, and astronomers have differed as to their tempera- 

 ture, some contending that they are much hotter than other parts of 

 the Sun, and others believing them to be comparatively cool. In 

 support of his theory that the chemical elements are broken up 

 into simpler constituents at very high temperatures, Lockyer adduced 

 observational evidence of a periodic change in the Sun-spot spec- 

 trum. At times of maximum solar activity, when spots are numer- 

 ous on the Sun, Lockyer found the most conspicuous lines in their 

 spectrum to be of unknown origin. Five or six years later, when 

 the solar activity had declined to a minimum, these lines seemed to 

 be replaced by the well-known lines of iron and other familiar sub- 

 stances. Lockyer accordingly concluded that at the maximum the 

 temperature of Sun-spots was sufficiently high to break up iron and 



' Maunder, Journal British Astronomical .IssociatioH, vol. xvii, p. 126. 



