34 8 -A Short History of Astronomy [CH. xii 



but was not able to carry out any extensive or systematic 

 measurements on this plan. With a view to the future 

 detection of such changes of brightness as have just been 

 mentioned, he Revised and carried out on a large scale 

 the extremely simple method of sequences. If a group of 

 stars are observed and their order of brightness noted at 

 two different times, then any alteration in the order will 

 shew that the brightness of one or more has changed. So 

 that if a number of stars are observed in sets in such a way 

 that each star is recorded as being less bright than certain 

 stars near it and brighter than certain other stars, materials 

 are thereby provided for detecting at any future time any 

 marked amount of variation of brightness. Herschel pre- 

 pared on this plan, at various times between 1796 and 1799, 

 four catalogues of comparative brightness based on naked- 

 eye observations and comprising altogether about 3,000 

 stars. In the course of the work a good many cases of 

 slight variability were noticed ; but the most interesting 

 discovery of this kind was that of the variability of the 

 well-known star a Herculis^ announced in 1796. The period 

 was estimated at 60 days, and the star thus seemed to form 

 a connecting link between the known variables which like 

 Algol had periods of a very few days and those (of which 

 Mira was the best known) with periods of some hundreds 

 of days. As usual, Herschel was not content with a mere 

 record of observations, but attempted to explain the observed 

 facts by the supposition that a variable star had a rotation 

 and that its surface was of unequal brightness. 



267. The novelty of Herschel's work on the fixed stars, 

 and the very general character of the results obtained, have 

 caused this part of his researches to overshadow in some 

 respects his other contributions to astronomy. 



Though it was no part of his plan to contribute to that 

 precise knowledge of the motions of the bodies of the solar 

 system which absorbed the best energies of most of the 

 astronomers of the i8th century whether they were 

 observers or mathematicians he was a careful and success- 

 ful observer of the bodies themselves. 



His discoveries .of Uranus, of two of its satellites, and of 

 two new satellites of Saturn have been already mentioned 

 in connection with his life ( 253, 255). He believed 



