β Persei (Algol) is another and perhaps the best known of all the variable stars. Its changes are very rapid, for it passes through its various gradations of brilliancy in less than three days. It was first noticed by Montanari in 1669, though it was left for Goodricke in 1782 to ascertain its period. The normal mag. of the star is 2·2, and it only shows distinct variation during the five hours which precede and follow a minimum, when it declines to 3·7 mag. Its period is shortening, for in 1782 it was 2^d 20^h 48^m 59^s·4, in 1842, 2^d 40^h 48^m 55^s·2, and at present Chandler finds it 2^d 20^h 48^m 51^s. As to the causes which contribute to these variations, they are invested in mystery. It has been conjectured that dark spots on the surfaces of the stars may, by the effects of rotation, introduce the observed alternations. Another surmise is that the temporary diminutions of lustre are to be ascribed to the interposition of dark satellites, and this theory seems tenable in regard to stars of the Algol type. It is satisfactory to note that a large amount of systematic work is being done in this important and delicate branch of research. Such stars as are subject to variation have been classed as follows:—1. Temporary or new stars; 2. Stars having long and pretty regular variation; 3. Stars irregularly variable; 4. Stars varying in short periods; 5. Stars of the type of Algol, which are liable to temporary diminutions of lustre. On the preceding page is a list of the most noteworthy variable stars. _List of Variable Stars._