^A 178 ANNUAL REPOET SMITHSONIAN INSTITUTION^ 1913. 



pendent investigations generallj^ agree to the seventh place of signifi- 

 cant figures. Also St. John and Ware have investigated the con- 

 sistency of the standards, each to each, by determining other wave 

 lengths independently by interpolation from several different stand- 

 ards, and are of the opinion that adjustments in the seventh place of 

 significant figures are hardly ever necessary, and will perhaps never 

 exceed 0.002 Angstrom units in any case. Investigations are now 

 on foot by St. John and Ware, Goos, Bums, and others to determine 

 a large number of tertiary standards of wave lengths intermediate 

 between these secondary standards, and it is hoped that good agree- 

 ment in regard to the tertiary standards will soon be obtained. 



It is found necessary in this work to specify the strength of the 

 electric current, the length of its arc, and the position of the slit of 

 the spectroscope with respect to the arc in order to get satisfactory 

 results. It now remains to go over the whole system of spectra of all 

 the chemical elements and determine the positions of their lines with 

 respect to these standard lines of iron, nickel, and barium which 

 have been adopted, and further to go over the whole solar spectrum 

 and to determine the position of its absorption lines with respect to 

 these standards. 



Although this will involve an enormous amount of careful work 

 in photography of the spectrum and in the measurements of the re- 

 sults, a work which will be so exacting as to appear at times almost 

 a drudgery to those who are engaged in it, yet like other good work 

 it is almost beyond question that it will yield unexpected fruits of 

 discovery in addition to those of investigations of the nature of the 

 sun and of the stars for which it is primarily undertaken. 



SOLAR PROBLEMS. 

 1. THE NATURE OF SUN SPOTS. 



Soon after the invention of the telescope, Galileo, in the year 1610, 

 observed spots on the sun. They continued to be observed by many 

 persons, and in the middle of the nineteenth century it was foimd 

 by Schwabe that the appearance of them was periodic. The average 

 interval between successive maxima or minima of sun spots is 11 

 years, but individual periods range from 8 years to 15 years in length. 

 The years from 1905 to 1910 were distinguished for large numbers of 

 sun spots, and the years 1910 to the present time for very small 

 numbers. We are now probably just at the beginning of a new sun- 

 spot maximum period, so that the report of spots being seen upon the 

 surface of the sun need not surprise us. Sun spots, as seen in the 

 telescope, consist of a dark central part called the umbra, and a less 

 dark shading around it called the penumbra. The appearance of the 

 sun when large spots are upon its surface is shown in the accom- 

 panying figure (pi. 1). 



