jects must not for a moment be confounded with clouds,
in the ordinary meaning of the word. The latter exist only suspended in
the atmosphere, while nebulae are immersed in the depths of space. Clouds
shine by the light of the sun, which they reflect to us; nebulae shine
with no borrowed light; they are self-luminous. Clouds change from hour
to hour; nebulae do not change even from year to year. Clouds are far
smaller than the earth; while the smallest nebula known to us is
incomparably greater than the sun. Clouds are within a few miles of the
earth; the nebulae are almost inconceivably remote.

Immediately after Herschel and his sister had settled at Slough he
commenced his review of the northern heavens in a systematic manner. For
observations of this kind it is essential that the sky be free from
cloud, while even the light of the moon is sufficient to obliterate the
fainter and more interesting objects. It was in the long and fine
winter nights, when the stars were shining brilliantly and the pale path
of the Milky Way extended across the heavens, that the labour was to be
done. The telescope being directed to the heavens, the ordinary diurnal
motion by which the sun and stars appear to rise and set carries the
stars across the field of view in a majestic panorama. The stars enter
slowly into the field of view, slowly move across it, and slowly leave
it, to be again replaced by others. Thus the observer, by merely
remaining passive at the eye-piece, sees one field after another pass
before him, and is enabled to examine their contents. It follows, that
even without moving the telescope a long narrow strip of the heavens is
brought under review, and by moving the telescope slightly up and down
the width of this strip can be suitably increased. On another night the
telescope is brought into a different position, and another strip of the
sky is examined; so that in the course of time the whole heavens can be
carefully scrutinised.

Herschel stands at the eye-piece to watch the glorious procession of
celestial objects. Close by, his sister Caroline sits at her desk, pen
in hand, to take down the observations as they fall from her brother's
lips. In front of her is a chronometer from which she can note the time,
and a contrivance which indicates the altitude of the telescope, so that
she can record the exact position of the object in connection with the
description which her brother dictated. Such was the splendid schem