ng, which gives us the
impression of movement without anything that moves it. Space is an
omnipresent, intangible, inconceivable nothing, outside of which
nothing which has existence can be even thought to exist. Let us now
try and get an insight into what we mean by perception of distance in
space.

The appreciation of distance depends upon what is called _parallax_,
or the apparent displacement of projectment of an object when seen by
our two eyes separately. If you hold up a finger and look at it, with
each eye separately, you will see that the finger is projected by each
eye on to a different part of the background; the angle which the
lines of sight, from each eye, make when they meet at the object, is
called the angle of parallax, and the further the object is away the
smaller that angle becomes; it is, in fact, the angle subtended, at
the object, by the distance between the two eyes. As the object is
brought nearer the eyes have to be inclined inwards to impinge on that
object; the appreciation of distance then, in our sense of sight, is
dependent upon our perception of the amount of inclination of those
two lines of sight, and is therefore an acquired knowledge. The
distance between the eyes is about 2-1/2 inches, and this is a very
short base line upon which to estimate distance; in fact, without the
help of perspective and known dimensions of surrounding objects, it is
doubtful if anyone could by its means estimate distance beyond a few
hundred yards. The object would, of course, also have to be an unknown
one, as, otherwise, the converse of the above comes into play, and the
distance could be estimated by the angle which the known diameter of
the object subtends at the eye; but this necessitates the size of the
object being known beforehand and the employment of perspective.

We can extend our perception of distances by, ourselves, moving from
one place to another, gaining thereby a longer base line, and noting
the displacement of projection of the object on a distant background;
by that means, distance up to several miles can probably be
appreciated. But, when we try to determine the distance of, say, the
Moon (240,000 miles away), we are helpless, especially as we have no
marked background, except in the case of occultations of the Sun or
Stars. But the Astronomer at once comes to our aid; a distance of
several miles is carefully measured on a level plane, and, by placing
telescopes at the extremities of