70 
PROFESSOR A. SCHUSTER ON THE PERIODICITIES OF SUNSPOTS. 
if n is the average number of spots in a group. The factor k depends on the 
instrument through which observations are conducted, and is taken to be unit)' for a 
telescope similar to the one used by Wolf. 
In Table I. " I give in separate columns for each year the mean daily sunspot area, 
as published in series (6), together with the mean annual value of Wolf’s numbers. 
The last column, giving the ratio between the two estimates of sunspot frequency, 
shows a discontinuity about the year 1860, the ratios after that date beino- markedlv 
higher than those before it. The discontinuity seems to coincide,with the time 
at which the areas began to be measured from photographs. I have therefore 
separately deduced the ratios for the different portions into which the complete series 
is divided. It is explained in publication (b), that Schwabe’s drawings were used 
up to the end of 1853, and from that date to the end of 1860 Carrixgtox’s drawings 
© 
were made use of in the reductions. For the year 1861 the gap between Carrington's 
series and the Kew series of photographs had to be filled by means of Schwabe’s 
drawings. The Kew series reaches into the early part of 1872, and as all measure¬ 
ments of areas from 1832 to 1872 were made under the direction of De la Kue and 
Stewart, we may conclude that there is no great amount of personal equation so 
far as measurement is concerned. The Greenwich series begins in July, 1873, the gap 
intervening between the Kew and Greenwich observations being filled by measure¬ 
ments of the Wilna photographs supplied by Professsor Backltjxd. 
Table II. shows the manner in which the ratio of Wolf’s numbers to the sunspot 
areas has varied according to the material on which the measurements were based. 
Table II. 
Series. 
Number of years. 
Mean area divided by 
AVolf’s numbers. 
Schwabe, 1832-1853 . . 
22 
9-71 
Schwabe, including 1861. 
23 
10-00 
Carrington, 1854-1860 . . . 
7 
10-79 
De la Rue, 1862-1871. . . . 
10 
16-92 
Greenwich, 1874-1901 . 
28 
13-38 
Total, 1832-1901. 
70 
12-50 
The last row gives the mean values as found from Table I. 
The larger comparative value obtained when the areas are obtained from photo¬ 
graphs is clearly shown by the table. It would seem therefore that Wolf’s series 
as a whole is more homogeneous than that obtained from the measurement of areas. 
A second fact brought out by Table I. is that the ratios in the Greenwich series are 
markedly higher at times of sunspot maxima. This shows that in Wolf’s method of 
* Tables I., IV., V., VI., 5 II. and VIII. will be found at-the end of the paper. 
