r in front of that bough, it has moved to a
position near the outline of the tree. This apparent displacement of the
strip of paper, relatively to the distant background, is what is called
parallax.

Move closer to the window, and repeat the observation, and you find that
_the apparent displacement of the strip increases_. Move away from the
window, and the displacement decreases. Move to the other side of the
room, the displacement is much less, though probably still visible. We
thus see that the change in the apparent place of the strip of paper, as
viewed with the right eye or the left eye, varies in amount as the
distance changes; but it varies in the opposite way to the distance, for
as either becomes greater the other becomes less. We can thus associate
with each particular distance a corresponding particular displacement.
From this it will be easy to infer that if we have the means of
measuring the amount of displacement, then we have the means of
calculating the distance from the observer to the window.

It is this principle, applied on a gigantic scale, which enables us to
measure the distances of the heavenly bodies. Look, for instance, at the
planet Venus; let this correspond to the strip of paper, and let the
sun, on which Venus is seen in the act of transit, be the background.
Instead of the two eyes of the observer, we now place two observatories
in distant regions of the earth; we look at Venus from one observatory,
we look at it from the other; we measure the amount of the displacement,
and from that we calculate the distance of the planet. All depends,
then, on the means which we have of measuring the displacement of Venus
as viewed from the two different stations. There are various ways of
accomplishing this, but the most simple is that originally proposed by
Halley.

From the observatory at A Venus seems to pursue the upper of the two
tracks shown in the adjoining figure (Fig. 47). From the observatory at
B it follows the lower track, and it is for us to measure the distance
between the two tracks. This can be accomplished in several ways.
Suppose the observer at A notes the time that Venus has occupied in
crossing the disc, and that similar observations be made at B. As the
track seen from B is the larger, it must follow that the time observed
at B will be greater than that at A. When the observations from the
different hemispheres are compared, the _times_ observed will enable the
lengths of the