However,
if he take note of a second mark and endeavour to keep it strictly in a
line with the first, he will easily keep a perfectly straight course. But
if he cannot find a second mark, it will not be difficult for him to use
the tufts of grass, the stones, or the other accidents of the soil, in
its place; they need not be precisely in the same line with the mark, but
some may be on the right and some on the left of it, in which case, as he
walks on the perspective of their change of position will be symmetrical.
Lastly, if he has not even one definite mark, but is walking among a
throng of forest trees, he may learn to depend wholly on the symmetry of
the changes of perspective of the trees as a guide to his path. He will
keep his point of sight unchanged and will walk in its direction, and if
he deviates from that direction, the want of symmetry in the change of
perspective on either side of the point on which he wishes to walk, will
warn him of his error. The appreciation of this optical effect grows
easily into a habit. When the more distant view happens to be shut out,
the traveller must regain his line under guidance similar to that by
which a sailor steers who only looks at his compass at intervals--I mean
by the aspect of the sky, the direction of the wind, and the appearance
of the forest, when it has any peculiarity of growth dependent on
direction.
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