IMPROVEMENTS IN INSTRUMENTS FOR MEASURING ANGLES BY REFLECTION
Year: 1883

page_text: PROVISIONAL SPECIFICATION left by John Henry Johnson at the Office
of the Commissioners of Patents on the 6th January 1883. Communicated
to him from Emile Hilaire Amagat a person residing at Paris in the Republic
of France..
JOHN HENRY JOHNSON of 47 Lincolns Inn Fields in the County of Middlesex
Gentleman "IMPROVEMENTS IN INSTRUMENTS FOR MEASURING ANGLES BY
REFLECTION a communication to me from Emile Hilaire Amagat a person resident
at Paris in the Republic of France
"
This invention relates to improvements in instruments for measuring angles by
reflection by means of which improvements it is rendered possible to measure the
angle formed by two distant points with the point of observation as well as
the projection in a horizontal plane of the said angle.
In principle the instrument to which this Invention is applied resembles the
ordinary box sextant but its construction and arrangement according to this
Invention render it essentially different from the latter as hereinafter described.
The improved instrument is provided with two mirrors hereinafter referred to as
the horizon glass and the index glass. The horizon glass is capable of turning
upon a horizontal axis and the index glass is capable of turning upon a vertical
axis situato in a line drawn from the eye of the observer to the distant point
viewed by direct vision. The rays from the point viewed by reflection impinge on
both the index glass and the horizon glass at the points where they intersect the
line of direct vision. The angle subtended by the two distant points is double tho
angle between the vertical planes of the two mirrors or glasses as in ordinary box
sextants. The angle which the two distant points make with the point of
observation is read off in degrees on a graduated scale provided with an index arın
connected to and moving with the mirrors-When the two glasses or mirrors are
parallel the index is at zero. In this instrument the reflected rays from the
horizon glass impinging on the eye is perpendicular to the plane of the said glass.
This condition is essential because these reflected rays must necessarily coincide as
hereinafter explained with the direct rays from the distant point or object whose
angle with the point viewed by reflection is to be ascertained.

page_text: In order to comply with this condition it is necessary that the vertical edge of
the index glass should coincide with its axis of rotation.
As the horizon glass is capable of turning upon a horizontal axis from this
· movement of rotation and from the condition that the line of direct vision is
perpendicular to the plane of the horizon glass it results that the reflected rays
become deflected in a vertical direction whilst the reflected rays and the eye of the
observer remain in the vertical plane of the line of direct vision.
Consequently if the eye be moved in a vertical plane and so placed as to view
directly any point in the same vertical plane by causing the direct visual rays to
pass over the upper or lower edge of the horizon glass the reflected image of any
point in the same vertical plane as the axis of the index glass may be brought to
coincide upon the edge of the horizon glass (after two successive reflections) with
the image of the point viewed by direct 'vision.
The precision of this instrument is very great even when the coincidence instead
of being precisely in the vertical plane is slightly to the right or to the left.