wn that the diameter of the Earth's orbit exceeded 183
millions of miles, and yet, with a base line of such enormous length,
and with instruments of the most perfect construction, astronomers were
only able to perceive the minutest appreciable alteration in the
positions of a few stars when observed from opposite points of the
terrestrial orbit.

It had long been the ambitious desire of astronomers to accomplish, if
possible, a measurement of the abyss which separates our system from the
nearest of the fixed stars. No imaginary measuring line had ever been
stretched across this region of space, nor had its unfathomed depths
ever been sounded by any effort of the human mind. The stars were known
to be inconceivably remote, but how far away no person could tell, nor
did there exist any guide by which an approximation of their distances
could be arrived at.

In attempting to calculate the distances of the stars, astronomers have
had recourse to a method called 'Parallax,' by which is meant the
apparent change of position of a heavenly body when viewed from two
different points of observation.

The annual parallax of a heavenly body is the angle subtended at that
body by the radius of the Earth's orbit.

The stars have no diurnal parallax, because, owing to their great
distance, the Earth's radius does not subtend any measurable angle, but
the radius of the Earth's orbit, which is immensely larger, does, in the
case of a few stars, subtend a very minute angle.

'This enormous base line of 183 millions of miles is barely sufficient,
in conjunction with the use of the most delicate and powerful
astronomical instruments, to exhibit the minutest measureable
displacement of two or three of the nearest stars.'--Proctor.

The efforts of early astronomers to detect any perceptible alteration in
the positions of the stars when observed from any point of the
circumference of the Earth's orbit were unsuccessful. Copernicus
ascribed the absence of any parallax to the immense distances of the
stars as compared with the dimensions of the terrestrial orbit. Tycho
Brahe, though possessing better appliances, and instruments of more
perfect construction, was unable to perceive any annual displacement of
the stars, and brought this forward as evidence against the Copernican
theory.

Galileo suggested a method of obtaining the parallax of the fixed stars,
by observing two stars of unequal magnitude apparently near to each
other, thoug