286 HERSCHEL. 



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 all either of very long 

 or very short periods. The illustrious astronomer con- 

 sidered that, by introducing between two groups that 

 exhibited very short and very long periods, a star of 

 somewhat intermediate conditions, for instance, one re- 

 quiring sixty days to accomplish all its variations of 

 intensity, he had advanced the theory of these phe- 

 nomena by an essential step ; the theory at least that 

 attributes every thing to a movement of rotation round 

 their centres which the stars may undergo. 



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 circum- 



o 



stance. I do not find, indeed, that the almost constant 

 association of two complementary colours (of yellow and 

 blue, or of red and green), ever led him to suspect that 

 one of those colours might not have any thing 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 apparent 

 colour ; but I have proved besides, that blue is incontes- 

 tably the colour of certain insulated stars, or stars that 

 have only white ones, or other blue ones in their vicinity. 

 Red is the only colour that the ancients ever distinguished 

 from white in their catalogues. 



Herschel also endeavoured to introduce numbers in 



