AN IMPROVED INSTRUMENT OR APPARATUS FOR ASCERTAINING THE DISTANCE OF AN OBJECT FROM AN OBSERVER, PARTICULARLY ADAPTED TO RANGE-FINDING OR USE AT SEA."
Year: 1880

page_text: PROVISIONAL SPECIFICATION left by William Henry Wheeler at the
Office of the Commissioners of Patents on the 1st September 1880.
WILLIAM HENRY WHEELER, of 100, High Street, Oxford. "AN IMPROVED
INSTRUMENT OR APPARATUS FOR ASCERTAINING THE DISTANCE OF AN OBJECT FROM
AN OBSERVER, PARTICULARLY ADAPTED TO RANGE-FINDING OR USE AT SEA."
By combining in one view the vision of a distant object from both ends,
respectively, of a constant portable base; to measure the angle contained within
these two lines of vision by the movement of an arm or radius, carrying a plane
mirror, or totally reflecting prism, adjusted over the centre on which it moves,
which mirror, while placed in one line of vision and allowing a direct view of any
object over it, reflects also rays from the same object as it would be seen along the.
other line of vision, they having been intercepted and bent towards it by another
mirror at the other end of the base, these mirrors having their reflecting surfaces
in one parallel plane, as with the index and horizon glasses of an ordinary sextant
when its index is at zero. Thus both lines of vision entering the eye together the
parallactic angle contained by them before reflection, and subtended by the base
line, is determined and measured by the movement over a divided arc of the arm
carrying the index mirror, after the following method:
A telescope (or plain sight in inferior instruments) is fixed at a right angle with
the base-line over the moveable arm carrying the index mirror, so as simultaneously
to admit through the object-lens the direct rays over the mirror and the reflected
rays from it. The telescope may be firmly fixed without its supports interfering
with the play of the index arm, for as this need only amount to half the parallactic
angle it will always be very small.
On directing this telescope in the line of direct vision from its end of the base
towards an object (preferably celesteal), whose distance is so great that its parallax
from the two ends of the constant base is insensible; the line of vision from the
other end of the base will be sensibly parallel with the first, and if the mirrors are
truly adjusted, the base line joining their centres being at a right angle with the
two sensibly parallel lines of sight, the two corresponding images will sensibly
coincide.
For the rays proceeding from the object along this second line of vision from
the other end of the base, having been reflected from a plane mirror or prism
placed so as to receive them at angle of 45°, proceed along the base line at a right

page_text: angle with their former course until they strike the second or index mirror on the
moveable arm, which being also placed to receive them at an angle of 45°, and with
its reflecting face towards the object-lens of the telescope, reflects them again at a
right angle into a line coincident with that of direct vision parallel with their
previous course. These two mirrors are adjustable, as in the sextant, to keep their
reflecting surfaces in a common plane at a right angle with the common plane of
the base and lines of vision, i.c., vertical where the the latter is horizontal, so
that the images may coincide vertically as well as horizontally.
The two visual images of the object, as they would be seen from the two opposite
extremities of the constant base, are now seen accurately to coincide where its
distance is so great that the parallax of the constant base is insensible. This absolute
coincidence in the observation of a celestial object is particularly valuable, as
furnishing a convenient fiducial test by which the instrumental index error may
be readily ascertained or corrected, the arm then marking zero on the divided
arc.
If now the adjusted instrument be directed to a distant terrestrial object not too
distant to allow of sensible parallax, its parallax is at once observed as a want of
coincidence in the images. For the two lines of vision which diverge from the
object to the two extremities of the base respectively, being no longer sensibly
parallel, the corresponding visual images will no longer sensibly coincide. But a
slight movement of the arm carrying the index mirror can restore coincidence by
deflecting the rays of the reflected line of vision until again sensibly parallel with
the direct line, and this movement over an angle which must be, according to
the well known laws of reflection, exactly one half of that which the constant
base line subtends at the object, will when read off a graduated are at once give the
required parallactic angle (one degree being divided and read off as two, as in the
ordinary sextant). This parallactic angle being thus observed and measured at one
operation and the base a constant quantity, it remains only to construct a table
for each length of base, by which the angle when read off may be at once reduced
to a corresponding distance of the object.
The accuracy of measurement with a firmly constructed instrument on this
design is limited only, first, by the optical perfection of the mirrors and telescope
employed, and secondly, by the accuracy and delicacy of the graduation, which
may be read off to extreme minuteness by a vernier or micrometer; for the
correctness of observation is independent of motion common to the whole instrument,
which may therefore, if desirable, be held in the hand or used from the deck of a
vessel like an ordinary sextant, and though the triangle is necessarily "ill-
conditioned," the methods of observation and measurement are so delicate as to
admit of the accurate measurement of an extremely small angle.