The physical evidence that our space is thus curved in higher space,
some have considered astronomy to furnish in what is called the
"negative parallax" of certain distant stars. This cannot be passed
by, though it is too deeply involved with the probable error of the
observers themselves to be considered more than an interesting fact
in this connection. Every one knows that the difference of angle
under which an object is seen from two standpoints is called its
parallax. The parallax of the stars--and the consequent knowledge of
their distance--is obtained by observing them from opposite points
of the earth's orbit around the sun. When a star is within measurable
distance, these angles are acute, and the lines from the star to the
earth at opposite sides of its orbit converge, therefore. But when
these lines, as sometimes happens, appear to be _divergent_, the
result is called a _negative_ parallax, and is explainable by higher
space relationships. Obviously, the divergence of the lines would
indicate that the object lies _behind_ the observer instead of in
front of him. This anomaly can be explained by the curvature of space
in the fourth dimension. If space is so curved, the path of light
itself is curved also, and a man--were his vision immeasurably keen,
not to say telescopic--could see the back of his own head! It is not
worth while to give this question of negative parallax too much
importance, by reason of the probability of error, but in this
connection it should be stated that there appears to be an undue
number of negative parallaxes recorded.