AN IMPROVED OPTICAL INSTRUMENT FOR MEASURING DISTANCES AND THE DIMENSIONS OF REMOTE OR INACCESSIBLE OBJECTS
Year: 1879

page_text: PROVISIONAL SPECIFICATION left by William Robert Lake at the Office of
the Commissioners of Patents on the 17th March 1879. A communication
from abroad by Edmond Landolt, of Paris, France, Doctor of Medicine.
WILLIAM ROBERT LAKE, of the Firm of Haseltine, Lake, & Co., Patent Agents,
Southampton Buildings, London. "AN IMPROVED OPTICAL INSTRUMENT FOR
MEASURING DISTANCES AND THE DIMENSIONS OF REMOTE OR INACCESSIBLE OBJECTS."
[A communication from abroad by Edmond Landolt, of Paris, France, Doctor of
Medicine.]
This Invention relates to an improved optical instrument or telemeter, by means
of which a person may ascertain what is the distance of a given point from the
spot where he is situated, and measure the dimensions of remote or inaccessible
objects.
.
The said Invention is based upon the principle of utilizing refraction through a
prism with variable angle composed of two elementary prisons of the same power
revolving one against the other at the same speed around a common axis, and in an
inverse direction. The said prisms are both provided with a central opening
coincident with the axis of rotation, and of which the surface is equal to half the
pupillary surface of the eye; or to be more exact, to half the sectional surface of the
luminous cone which enters the pupil of the eye, so that the observer who looks
through the centre of the instrument sees at the same time through the opening
and through the mass of glass forming the prisms. Providing the latter with
openings permits the said arrangement of the rotating prisms to be applied
to telemetrical measuring instruments, and forms the chief feature of this
Invention.
In carrying the said Invention into practice I provide a circular box or case of
metal within which the prisms are mounted. Each prism is secured within a ring
or circle, and is mounted upon discs, which discs are adjusted so as to have but
little friction in each face of the said box or case. Each of the said discs is in
connection with a crown wheel, both of these wheels gearing with a common pinion
whose axis extending towards the centre of the box turns in a bearing in the
circumference of the said box. By means of this arrangement the turning of a
knob provided on the spindle of the said pinion causes a rotatory motion to be
communicated to the two discs carrying the prisms which revolve in a contrary
direction. In the initial position of the parts the prisms are side by side, their

page_text: exterior surfaces being parallel, and the observer sees objects naturally, because in
this position the effect of the prisms is neutralized; but immediately the prisms
are caused to rotate their exterior surfaces incline at a constantly increasing angle,
the part of the rays of light which passes outside the opening is separated, and the
object observed becomes divided. As the rotation continues the two images continue
to get farther apart until the maximum distance of separation is attained which the
instrument is capable of producing, the said maximum distance being attained when
the apices coincide..
It will be understood that by the aid of an object of known dimensions and by
rotating the prisms sufficiently to bring the two images of this object into a given
position, for example into such a position that they-touch at- their opposite edges,
the distance between the object and the instrument may be determined.
When there is one single object, as for instance a man, a horse, or a tree, it is
obvious that it will be necessary to rotate the prisms a less distance the more
distant the object is from the instrument. The distance will be given by a table 1
with two entries, and upon which will be inscribed the numbers indicating this
distance corresponding to the angle of rotation of the prisms, which angle will be read
upon a graduated scale engraved upon one of the faces of the said circular box or
case.
I attach to the said box or case an astronomical or other telescope, which assists
the observer in seeing the object, and which he can adjust to his sight.
This box or case will be furnished with a detachable handle, or with a foot or
stem to be attached at will for holding the instrument in the hand.
When the said instrument is employed for measuring the dimensions of an object
situated at a known distance, another table will be employed indicating the 2
dimensions sought for corresponding with the angle of rotation.
The said apparatus may be constructed of any other suitable form, and the
details may be somewhat varied without departing from the principle of the said
Invention.