CHAPTER I. 



The Material of Observation and its first Treatment. 



All the material employed in this paper is published in the »G re e n wi c h Ob- 

 servations» which appear yearly. The observations here published consist of measure- 

 ments of solar photographs taken at Greenwich, Dehra Dun and at the Kodaikanal Ob- 

 servatory in India, and at the Royal Alfred Observatory of Mauritiu't. In collecting 

 the material from all these observatories it has 

 been possible to obtain photographs of the sun 

 almost every day during the last 3Ö years. 



Table V shows us the number of pho- 

 tographs received each year. From 1873 — 1881 

 all the photographs are taken at Greenwich. 

 Thus these years are rather incomplete. Du- 

 ring the time from 1882—1884 the photo- 

 graphs are from Greemvich and Dehra Dim, 

 Northwest Provinces, India; 1885 — 1903 from 

 Greemvich, Dehra Dim and the Royal Alfred 

 Observatory of Mauritius. Since the year 1904 

 the solar photographs are also received from 

 the Kodaihanal Observatory in Southern India. 



The Greenwich photographs are either 

 taken with the Thompson or with the Dall- 

 meyer Photoheliograph. The first of these 

 instruments gives a solar image of about 7.6 

 inches, while the latter gives an image of 

 nearly 8 inches on the photographic plate. At Dehra Dim as well as the Kodai- 

 Jcanal Observatory Dahmeyer Photoheliographs are employed, which also give images 

 of about 8 inches. 



Owing to the kindness of Mr. Dyson, the Royal Astronomer, I am able to give 

 a reproduction of one of these plates (PI. I). A sun-spot of the size shown in this 

 figure is naturally of very rare occurrence. Its area is thus proved to be 2771 millionths 

 of the visible hemisphere of the sun. 



TABLE V. 

 Days with Solar Photographs. 



Days with 

 Photo- 

 graphs 



Days with 

 Photj- 

 graphs 



1873 

 1874 

 1875 

 1876 

 1877 

 1878 

 1879 



1893 

 1894 

 1895 

 1896 

 1897 

 1898 

 1899 

 1900 

 1901 

 1902 

 1903 

 1904 

 1905 

 1906 

 1907 

 1908 

 1909 

 1910 



