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 cireum- 
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 
