HERSCHEL. 213 



occasionally directing liis telescopes to this mysterious star. At botli 

 tlioseepoclis the observer's attention was chiefly directed to the absolute 

 values of the maxima and muiim'i of intensity. 



Tlie changeable star in the Whale was not the onlj' periodical star 

 with which Herschel occupied himself. His observations of 1795 and of 

 1796 i)roved that a Herculis also belongs to the category of variable 

 stars, and that the time requisite for the accomplishment of all the 

 changes of intensity, and for the star's return to any given state, was 

 sixty days and a quarter. When Herschel obtained this result, about 

 ten changeable stars were already known ; but they were ail either of 

 very long or very short periods. The illustrious astronomer considered 

 that by introducing between two groups that exhibited very short and 

 very long loeriods a star of somewhat intermediate conditions — for in- 

 stance, one requiring sixty days to accomplish all its variations of inten- 

 sity — he had advanced the theory of these phenomena by an essential 

 step ; the theory at least that attributes all to a movement of rotation 

 which the stars may undergo round their centers. 



Sir William Herschel's catalogues of double stars offer a considerable 

 number to which he ascribes a decided green or blue tint. In binary 

 combinations, when the small star appears very blue or very green, the 

 large one is usually yellow or red. It does not appear that the great 

 astronomer took sufficient interest in this circumstance. I do not find, 

 indeed, that the almost constant association of two complementary colors 

 (of yellow and blue, or of red and green) ever led him to suspect thnt 

 one of tliose colors might not have anything real in it, that it often might 

 be a mere illusion, a mere result of contrast. It was only in 1825 that 

 I showed that there are stars whose contrast really explains their appar- 

 ent color ; but I have proved besides that blue is incontestably the color 

 of certain insulated stars, or stars that have only white ones, or other 

 blue ones, in their vicinity. Eed is the oidy color that the ancients ever 

 distinguished from white in their catalogues. 



Herschel also endeavored to introduce numbers in the classification of 

 stars as to magnitude; he has endeavored, by means of these, to 

 show the comparative intensity of a star of th^ 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 

 Avhen the best telescopes are used. Diameters estimated by seconds — 

 that is to say, reduced according to the magnifying power — diminish in 

 certainty as the magnifying ])ower 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 j)rofound mathematical knowledge, and a peculiar tact. 



