LABOURS IN SIDEREAL ASTRONOMY. 287 



the classification of stars as to magnitude ; he has en- 

 deavoured, by means of numbers, to show the compara- 

 tive intensity of a star of first magnitude, with one of 

 second, or one of third magnitude, &c. 



In one of the earliest of Herschel's memoirs, we find, 

 that the apparent sidereal diameters are proved to be for 

 the greater part factitious, even when the best made tele- 

 scopes are used. Diameters estimated by seconds, that 

 is to say, reduced according to the magnifying power, 

 diminish as the magnifying power is increased. These 

 results are of the greatest importance. 



In the course of his investigation of sidereal parallax, 

 though without finding it, Herschel made an important 

 discovery ; that of the proper motion of our system. To 

 show distinctly the direction of the motion of the solar 

 system, not only was a displacement of the sidereal 

 perspective required, but profound mathematical knowl- 

 edge, and a peculiar tact. This peculiar tact Herschel 

 possessed in an eminent degree. Moreover, the result 

 deduced from the very small number of proper motions 

 known at the beginning of 1783, has been found almost 

 to agree with that found recently by clever astronomers, 

 by the application of subtile analytical formulas, to a con- 

 siderable number of exact observations. 



The proper motions of the stars have been known and 

 proved for more than a century, and already Fontenelle 

 used to say in 1738, that the sun probably also moved 

 in a similar way.' The idea of partly attributing the 

 displacement of the stars to a motion of the sun, had sug- 

 gested itself to Bradley and to Mayer. And Lambert 

 especially had been very explicit on the subject. Until 

 then, however, there were only conjectures and mere 



