MARY WHITNEY. 209 



was a large round nucleus, a photograph of which I 

 have brought with me. But I should say from my ob- 

 servation that the striking groups appeared before the 

 period of maximum activity. 



As I have before said, the sun-spots of the past two or 

 three years have been of interest to astronomers because 

 the period of sun-spot activity has culminated during 

 this time. So far, however, have the phenomena varied 

 from those predicted that astronomers differ as to their 

 bearing upon theory. 



The sun-spots were first systematically observed by 

 Schwabe, of Dessau. During the years from 1826 to 

 1868 he counted the spots and made drawings. In 1851 

 he published the results of twenty-five years of obser- 

 vation. 



Carrington's observations began later and covered 

 several years. These were made with more exactness 

 than Schwabe' s, and gave also change in latitude and 

 longitude of spots. Beside these valuable series, there 

 are the observations of Spoerer, at Potsdam, and those 

 of Secchi. The solar photographs taken at the Kew 

 Observatory have also been made use of in determining 

 the period of variability. In the earlier observations 

 the spots were counted only. Later investigators have 

 measured the areas, on the ground that the number of 

 spots is not so true a measure of solar activity as the 

 amount of surface disturbed. 



De la Rue gave great labor to the measurement of 

 areas in the Schwabe and Carrington series, and the 

 Kew photograydis were also measured in this way. The 

 areas of umbra and penumbra are measured separately, 

 and correction is made for the foreshortening of spots 

 seen obliquely. Although these careful modern obser- 

 vations cover a considerable number of years, the time 

 is yet too short for a confident determination of the 

 period of variability. 



193 



