wished it to answer.

Again and again, in his various papers, he returns to the question of
the _limit of vision_. As BESSEL has said:

"The naked eye has its limit of vision in the stars of the sixth
magnitude. The light of fainter stars than these does not affect the
retina enough for them to be seen. A very small telescope penetrates
to smaller, and, in general, without doubt, to more distant stars.
A more powerful one penetrates deeper into space, and as its power is
increased, so the boundaries of the visible universe are widened,
and the number of stars increased to millions and millions. Whoever
has followed the history of the series of HERSCHEL'S telescopes will
have observed this. But HERSCHEL was not content with the bare fact,
but strove ever to know _how far_ a telescope of a certain
construction and size could penetrate, compared with the naked and
unassisted eye. These investigations were never for the discovery of
new facts concerning the working of his instruments; it was for the
knowledge of the distribution of the fixed stars in space itself
that he strove. . . . HERSCHEL'S instruments were designed to aid
vision to the last extent. They were only secondarily for the taking
of measures. His efforts were not for a knowledge of the _motions_,
but of the _constitution_ and _construction_ of the heavenly
bodies."

Besides the stands for his telescopes, which were both ingenious and
convenient, HERSCHEL devised many forms of apparatus for facilitating
the art of observation. His micrometers for measuring position angles,
his lamp micrometer, the method of limiting apertures, and the methods
he used for viewing the sun may be mentioned among these.

Points in practical astronomy are considered all through the years of
observation. A reference to his original papers will show how numerous,
how varied, and how valuable these are. I cannot forbear quoting here
the account of a precaution observed during his examination of the belts
on _Saturn_ (1794).

It is the most striking example of how fully HERSCHEL realized that the
eye of the observer is a material part of the optical apparatus of
astronomy. Simple as this principle may appear, it was an absolute
novelty in his day.

In making these observations, he says:

"I took care to bend my head so as to receive the picture of the
belt in the same direction as I did former