Page:Sir William Herschel, his life and works (1881).djvu/153

Rh The example of Mira Ceti and of other known variables makes this at least doubtful. But the sun itself may vary for all that we know. It is a simple star like the rest.

This question of variability in general is an important one, then. It can only be tested by making accurate catalogues of the relative brilliance of stars at various times, and by comparing these. No such general catalogue existed before time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he made from observation a series of four catalogues, in which were set down the order of sequence of the stars of each constellation.

The method adopted by was perfectly simple in principle, though most laborious in practice. Suppose any number of stars, A, B, C, D, E, ... etc., near enough to each other to be well compared. The process consists simply in writing down the names of the stars, A, B, C, etc., in the order of their relative brightness. Thus if for a group of eight stars we have found at one