Improved Method of Obtaining Solar Altitudes by the Aid of Instantaneous Photography.
Year: 1891

page_text: Date of Application, 15th June, 1891
Complete Specification Left, 9th Mar., 1892-Accepted, 14th May, 1892
PROVISIONAL SPECIFICATION.
Improved Method of Obtaining Solar Altitudes by the Aid of
Instantaneous Photography.
1, JOHN JOLY Civil Engineer of 39 Waterloo Road Dublin, Ireland, do hereby
declare the nature of this invention to be as follows:-
The method consists in photographing instantaneously the sun and horizon so
that from measurements made subsequently upon the plate the angular distance of
the sun and the horizon at the moment of taking the photograph can be ascertained.
I preferably carry out my invention by affixing to a Hadley's sextant a photographic
apparatus replacing the usual eye-piece or telescope, which when the moveable
limb of the sextant is so set that the sun and horizon are brought into approximate
coincidence and the observer, through a suitable sighting glass, points the instru-
inent at the sun or horizon, then, upon releasing a shutter, the photograph taken of
the direct and reflected images shows the exact want of coincidence of these and
so affords a correction which applied to the setting of the sextant gives the true
altitude. To fix the time of observation accurately I may simultaneously
photograph the dial of a time piece.
COMPLETE SPECIFICATION.
Improved Method of Obtaining Solar Altitudes by the Aid of
Instantaneous Photography.
I JOHN JOLY Civil Engineer of 39 Waterloo Road, Dublin, do hereby declare
the nature of this invention and in what manner the same is to be performed,
to be particularly described and ascertained in and by the following statement :-
The method consists in photographing simultaneously the sun and horizon so
that from measurements made subsequently upon the photographic plate the
angular distance of the sun from the horizon at the moment of taking the
photograph can be ascertained. To carry out this invention I preferably use any
one of the following arrangements :-(1). The observing telescope of an ordinary
Hadley's sextant is replaced by a photographic camera of the following con-
struction. The photographic lens is carried at the end of a straight tube A
Fig. which replaces the telescope tube and is of such similar dimensions as to be
readily inserted in its stead when the telescope is removed. At that end of the
tube which is nearest the observer when using the sextant a small light-tight
chamber B is attached, of wood or metal or other suitable substance, serving as
the body of the camera which receives the dark slide or back carrying the photo-
graphic plate, R. The image formed by the lens L is thrown upon this plate and
the plate may be uncovered within the camera by drawing aside a sliding cover.
Or the plate may be inserted in the camera and closed up secure from light within
it without the use of a back and by effecting the insertion of the plate in a dark
place. When it is required to make exposure a shutter S of any of the usual
forms and inserted in the tube which carries the lens or at either extremity of the
tube is actuated by a spring catch being released or by pneumatic or electric
means. It will be understood that when the observer is using the instrument he
must hold it so that the lens is directed at the horizon at a point vertically beneath
the sun so that the horizon seen through the clear part of the half-silvered mirror
of the sextant is imaged upon the photographic plate within the camera while
i Pe

page_text: :
If
simultaneously the image of the sun reflected first from the moveable mirror of
the sextant and secondly from the silvered part of the half-silvered mirror of the
sextant is thrown upon the photographic plate. When the photograph is taken,
therefore, both the sun and horizon are photographed upon the plate. Hence it is
necessary that the observer possess a means of telling when he is holding the
instrument so that the reflected image of the sun and the direct image of the
horizon are formed upon the plate. To effect this in this form of the instrument a
small sighting tube T is rigidly attached to the camera. This points parallel to
the axis of the tube carrying the lens and may or may not be provided with a lens
or lenses to help the vision of the observer. Looking through this as if making
an observation in the ordinary way the observer obtains an approximate or perfect
coincidence of sun and horizon by moving the moveable limb of the sextant. At
the proper moment he releases the shutter noting the time of day in the usual way.
The reading of the moveable limb being now noted and the plate subsequently
developed he measures upon the latter the want of coincidence, if any, of the sun
and horizon and knowing the angular divergence in degrees, minutes or seconds of
an arc corresponding to the measured want of coincidence adds or subtracts the
amount to or from the reading of the moveable limb of the sextant. It will be
understood that distances in the plate may be scaled by any convenient and accurate
measuring instrument to a close degree, and that the corresponding angular values
are determined readily once for all for each instrument by a few direct observations
upon objects of known dimensions placed at known distances from the camera.
the camera is so large that the sighting tube rigidly attached to one side of it
fails to catch the reflection from the half-silvered mirror the line of sight falling
outside it (a) I replace the fixed mirror of the sextant by one of suitable dimensions,
or (6) I affix rigidly a reflecting right angled prism P Fig. 2 to the end of the
sighting tube T and a second Qopposite to it upon the tube of the camera or
within it so that the line of sight is first bent at right angles at the extremity of
the sighting tube and by this means directed into the prism attached to the camera
tube when it is again bent at right angles so that it is redirected parallel to its
direction in the sight tube but is now close in to the tube of the camera, or
within it, so as to cover the same field of view embraced by the camera lens. (c) Or
the two reflecting prisms (which also may be mirrors set at 45° to the line of sight)
may be carried by the shutter which serves to expose the plate in such a manner
that entering a slot in the camera or camera-tube A Fig. 3 one of them P serves
to reflect out all the rays which come from the photographic lense into the sight
tube T and so by the second reflection into the eye of the observer. The shutter
is actuated by causing the prism within the camera to move rapidly to one side so
as to uncover for a moment the photographic plate R to the rays which immediately
before were entering the eye of the observer. By a return movement, if desired,
the rays may again be redirected into the sighting tube and the plate thus again
obscured, or the prism or reflector continuing its travel in the one direction after release
may be provided with an attached, perforated opaque screen, N, prolonging it so
that the perforation in its passage across the axis of the tube merely admits the
light for a moment. In any of these ways may secure that the observer is able
to direct the camera with certainty at the point on the horizon vertically beneath
the sun.
To avoid solarization of the image of the sun upon the plate I take
into use one or more of the dark coloured glasses attached to the sextant or replace
one or more of these by tinted glass specially suited to the purpose.
(2) Or secondly I may carry out my invention as follows:-The telescope of
the sextant is retained, with modification of the optical character of its object
glass or eye piece if necessary, but these remain such that the observer looking
through the telescope sees a clear image of the sun and horizon in the usual manner.
Entering the tube of the telescope, A Fig. 4, a reflecting prism P or mirror reflects
out of the telescope part of the rays which converge from the object glass, L,
into a small camera, C, of construction similar to that already described affixed to
the side of the telescope. A suitable shutter S between the camera and the

page_text: telescope or in front of the reflector within the telescope enables the plate K to
be kept obscured till the observer desires to expose it. The prism or mirror used
to reflect the rays into the camera may be carried rigidly attached to the extremity
of a small tube prolonging the camera so that the observer may withdraw the
prism with the camera readily, if desired, using the sextant then quite in the
ordinary manner. Or the camera may readily be taken into use by passing
the prism and tube of the camera through the perforation provided in the side
of the telescope tube when the tube of the camera is held firmly in a short pro-
It will be understood that the
jecting collar attached to the telescope tube.
sighting in this case calls for no further provision. To secure a proper image of
the sun and horizon upon the plate 1 may use a supplemental lens either forming
part of the reflecting prism or inserted within the short tube attached to the
camera. Or in place of having the reflector fixed in position I may arrange to
have the reflector P Fig. 5 moveable so that it forms part of the shutter used for
exposing the plate instantaneously. In this case it may be fixed above the
orifice of the shutter as in the figure or so connected with the shutter that when
this orifice is in the position for uncovering the plate as in the figure the prism
finds itself carried moving perpendicularly to the paper in Fig. 5 into the path of
the rays within the telescope A deflecting these through the orifice of the shutter
upon the plate, R. In this way I may arrange that the observer before actuating
the shutter receives all the light and at the moment in which the photograph is
being taken all the light is deflected into the camera. Or I may arrange by
limiting the surface of the reflector that in the latter position some of the rays still
enter the eye of the observer.
(3) I may also carry out my invention as follows:-The principle of the
sextant being retained (i.e. that the angle between the first and last directions of a
ray which has undergone two reflections in the same plane is equal to twice the
inclination of the reflecting surfaces to each other) I modify the construction of
the instrument in the following manner. A camera in the form of a box or tube A
Fig. 6 carrying a photographic lens L and allowing of a sensitive plate R being
exposed within it by an instantaneous shutter S is provided with the usual partly-
silvered mirror m of the sextant rigidly attached at a convenient angle in front of
the lens. Above the camera a second mirror n, corresponding to the moveable
mirror of the sextant, is carried in a limb / moving over a graduated arc, a, fixed
The moveable mirror is con-
rigidly to the camera, in the manner of the sextant.
veniently placed so that a reflection of the sun from the mirror may be directed
upon the fixed partially-silvered mirror in front of the camera and so reflected
into the camera, the index hand being rigidly attached to the axis of the mirror
and moved in the ordinary manner. A slow-motion tangent screw to secure
accurate setting of the mirror may be provided in the usual manner, as well as
suitably tinted glasses to screen off some of the sun's light as in the ordinary
Hadley's sextant. It will be understood that in this form of my invention the
camera is not a subordinate but a principal part of the instrument the object being
to permit the use of a larger lens and so enable an image of the horizon to be
photographed in dark or obscure weather. The divided arc instead of carrying
the camera as in the previous forms described in this Specification may be considered
as carried by the camera. In other respects the use of the instruments is alike.
To facilitate the pointing and direction of the instrument I provide a sighting
tube T or small telescope which by any of the arrangements serving to effect the
same and as specified in the first previously described form of my invention
enables the observer to point the instrument at the correct point upon the horizon.
In all these instruments I may use square or circular photographic plates or of
such shape as may be found most convenient, or sensitive films of such character
as will not interfere with the accuracy of the measurements made upon them by
shrinkage or distortion when being developed. I may use any lens or train of
lenses suitable to form a clear image upon the plate and suitable adjustments for
focus more especially in conjunction with the eye piece.

page_text: Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is :-
:-
1. The combination of instantaneous photography with the Hadley's sextant
enabling the observation of solar altitudes at sea to be obtained with increased
ease and accuracy.
2. The combination of a photographic camera with an instrument of the nature
of the Hadley's sextant for measuring angles by reflecting the image of one distant
object into coincidence with another and photographing the more or less coincident
images in the camera, so that subsequently upon the developed plate a want of
coincidence may be detected and allowed for as described in the foregoing.
3. The use of a moving reflector and shutter in the foregoing instruments so that
the light may be at will directed into the eye of the observer or wholly or in part
instantaneously upon the photographic plate.
4. The use of a fixed reflector in the foregoing suitably located so that the light
is partly directed into the eye of the observer and partly transmitted to the sensitive
plate, which is exposed to it by an instantaneous shutter.
Dated this Third day of March 1892.
JOHN JOLY.