cernible way of the twilight, striking up towards the
Pleiads, and seeming almost to touch them. It is so observed any clear
night, but it is best _illae nocte_. There is no such way to be
observed at any other time of the year. But what the cause of it in
nature should be, I cannot yet imagine, but leave it to further
inquiry.' The further inquiry followed soon afterwards, for Cassini,
the eminent French astronomer, having carefully observed the
phenomenon from 1683 to 1688, communicated the results to the Academie
des Sciences. Some of his views and determinations were well founded;
and from them we gather that the zodiacal light was nearly or quite
the same in his day as at present. Others also devoted considerable
attention to it, and noticed the variations in brightness in different
years, which subsequent observations have verified. Since then, it has
been made more or less a subject of investigation by modern
astronomers, and has been observed in many parts of the world; the
first observations in the southern hemisphere being those made by
Professor Smyth at the Cape of Good Hope, from 1843 to 1845. In that
latitude, the zodiacal light is best seen in spring evenings, at an
angle of 30 degrees, visible long after sunset; its opposite peak is
discernible at daybreak, but has scarcely come into view before the
rising sun overpowers it. In autumn, the reverse takes place; the best
appearance is in the morning.

To understand what is meant by the 'opposite peak,' we are to regard
the zodiacal light, of which we see only one end in our latitudes, as
a body extending all round the sun in the same form, presenting at a
distance the appearance of one of those flat elongated oval nebulae
seen in the heavens. Its direction is at right angles to that of the
sun's rotation, a straight line drawn from either pole of the great
luminary divides it in the centre. From its outline resembling that of
a lens in section, it is frequently described as a 'cosmical body of
lenticular form.'

From this account of what the zodiacal light appears to be, we proceed
to consider what it is. Some inquirers--arguing from the 'nebular
theory,' which assumes the formation of the several planets, one after
another, from nebulous matter--have supposed the zodiacal light to be
a remnant of that matter yet unconcentrated. In this view, it may be a
nebula, brightest in the centre, as is the case with most, and fainter
towards the margin. According