t intervals of time, however,
at rising or setting, one or other would be visible alone; and the
phenomena of sunset and sunrise must therefore be very varied, and also
exquisitely beautiful, in worlds circling round such double suns. But
when each sun has a separate system, even more remarkable relations must
be presented. For each system of dependent worlds, besides its own
proper sun, must have another sun--less splendid, perhaps (because
farther off), but still brighter beyond comparison than our moon at the
full. And, according to the position of any planet of either system,
there will result for the time being either an interchange of suns,
instead of the change from night to day, or else double sunlight during
the day, and a corresponding intensified contrast between night and day.
Where the two suns are very unequal or very differently coloured, or
where the orbital path of each is very eccentric, so that they are
sometimes close together and at others far apart, the varieties in the
worlds circling round either, or around the common centre of both, must
be yet more remarkable. "It must be confessed," we may well say with Sir
John Herschel, "that we have here a strangely wide and novel field for
speculative excursions, and one which it is not easy to avoid
luxuriating in."'

Anyone who takes a cursory glance at the heavens on a clear night can
readily perceive that there exists considerable diversity of colour
among the stars. The contrast between some is pronounced and well
marked, whilst others exhibit refined gradations of hue.

The most numerous class of stars are those which are described as white
or colourless. They comprise about one-half of the stars visible to the
naked eye. Among the most conspicuous examples of this type are
Sirius--whose diamond blaze is sometimes mingled with an occasional
flash of blue and red--Altair, Spica, Castor, Regulus, Rigel, all the
stars of Ursa Major with the exception of one, and Vega--a glittering
gem of pale sapphire, almost colourless. The light emitted by stars of
this class gives a continuous spectrum, the predominating element being
hydrogen, having a very elevated temperature and under relatively high
pressure. The vapours of iron, sodium, magnesium, and other metals, are
indicated as existing in small quantities.

The second class of stars is that to which our Sun belongs. They are of
a yellow colour, and embrace two-thirds of the remaining stars. The most
pro