THE LIGHT OF THE STARS. 177 



mathematics in the Lawrence Scientific School 

 at Harvard, after a distinguished university 

 career. In 1876 he succeeded Winlock as 

 director of the Harvard Observatory, and in the 

 following year he commenced his photometric 

 studies. He invented an instrument named the 

 meridian photometer, with the aid of which he 

 succeeded in determining, in the years 1879 to 

 1882, the exact brilliance of 4260 stars to the 

 sixth magnitude between the north celestial pole 

 and thirty degrees of south declination. At a 

 later date he devised a larger photometer, with 

 which he made over one million observations. 

 Pickering next extended his survey to the 

 southern hemisphere, erecting the photometer on 

 the slope of the Andes, where the Harvard 

 auxiliary station at Arequipa is now located, 

 and where 8000 determinations of stellar brilli- 

 ance were made. Meanwhile Pritchard, at Ox- 

 ford, published in 1885 his * Uranometria Nova 

 Oxoniensis,' with photometric determinations of 

 the brilliance of 2784 stars from the pole to 

 ten degrees of south declination. Both of 

 these catalogues were epoch-making works, and 

 testify to the enthusiasm and perseverance of 

 the astronomers who designed them. 



The study of stellar photometry glides into 

 that of stellar variation. At the beginning of 



M 



