London
and inserted in the _Philosophical Transactions_ treats precisely of the
changes of intensity of the star _o_ in the neck of the Whale.

This memoir was still dated from Bath, May, 1780. Eleven years after, in
the month of December, 1791, Herschel communicated a second time to that
celebrated English Society the remarks that he had made by sometimes
directing his telescopes to the mysterious star. At both those epochs
the observer's attention was chiefly applied to the absolute values of
the _maxima_ and _minima_ of intensity.

The changeable star in the Whale was not the only periodical star with
which Herschel occupied himself. His observations of 1795 and of 1796
proved 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 all 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 periods, a star of somewhat intermediate conditions,--for instance,
one requiring sixty days to accomplish all its variations of
intensity,--he had advanced the theory of these phenomena 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 circumstance. 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 incontestably 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 dist