IMPROVEMENTS TRANSIT INSTRUMENTS
Year: 1882

page_text: LETTERS PATENT to Josiah Latimer Clark of 6 Westminster Chambers
Victoria Street Westminster for an Invention of "IMPROVEMENTS
TRANSIT INSTRUMENTS
"
IN
PROVISIONAL SPECIFICATION left by the said Josiah Latimer Clark
at the Office of the Commissioners of Patents on the 18th November 1882.
JOSIAH LATIMER CLARK of 6 Westminster Chambers Victoria Street Westminster,
* IMPROVEMENTS IN TRANSIT INSTRUMENTS"
The object of these improvements is to reduce the cost, and facilitate the use of
the transit instrument.
My first improvement consists in a plan by which the spirit level on the
declination circle may be dispensed with. I effect this by fixing the altitude or
declination circle on the main axis of the instrument, but the vernier or index
instead of being adjusted by a spirit level or other gravity pointer, is adjusted by
means of an arm or prolongation, which rests against a fixed or adjustable stop
fixed on the solid frame of the instrument, so placed that when the arm rests
against the stop, the index is in its zero position, as it would be if a level or
gravity pointer were employed-this arrangement may with great advantage be
used in conjunction with a spirit level.
My second improvement consists in a method of securing the frame of the
instrument to the solid base on which it is fixed so as to allow full play for the use
of the levelling screws. I effect this by placing beneath the frame a flat metal
spring which is secured to the under side of the instrument by two or more screws
at each end which screw upwards into the base-while at the centre of the frame
a single screw passes through a hole in the spring and is screwed down into the
solid base on which the instrument rests-the elasticity of the spring then allows
full play for the adjustment of the levelling screws without tending to distort the
frame work of the instrument.
My third improvement consists in a peculiar cap to the telescope; this cap is of
the ordinary form, but carries a hinged circular lid which when closed, shuts off the

page_text: whole aperture of the telescope except a small circular opening in the centre having
a diameter of about a quarter of an inch. The inside of this lid is whitened by
paint or paper, which when placed at a proper angle and illuminated by a lantern
casts a light down the tube of the telescope and renders the cross hairs visible.
When closed it serves a different purpose and enables a meridian mark to be clearly
focussed, at a short distance such as thirty or forty yards which is often of great
advantage.
My fourth improvement consists of a support for the lamp which illuminates the
wires. This support consists of an upright stem inserted in a base at one side of the
instrument, carrying two pairs of parallel arms, supporting a tube, into which the
stem of an ordinary spring candle lamp is inserted-these arms work up and down
on centres and are both of the same length so that the table moves in a circle but
at the same time always remains upright. These arms are of the same length as
the front half of the telescope, and the centre of the upper pair of arms is in a
line with the axis of the instrument, so that the lamp moves in a circle and is
always held opposite the illuminated lid above described.
The lamp carries a hinged reflector for throwing the light downward upon a
watch while observing.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Josiah Latimer Clark in the Great Seal Patent Office on the 18th
May 1883.
JOSIAH LATIMER CLARK of 6 Westminster Chambers, Victoria Street
Westminster "IMPROVEMENTS IN TRANSIT INSTRUMENTS
"
The object of these improvements is to reduce the cost of the transit instrument
and to facilitate its use.
To effect this I make several modifications. I fix an arm and adjusting screw
on the axis of the telescope, or rather on the vernier plate which turns on the
axis of the telescope, which arm resting against a stop on some part of the
framework of the instrument, serves as a fixed zero point, and enables one to
dispense entirely with the spirit level usually employed for setting the declination
circle level, or if preferred, it may be used in conjunction with it.
In Figure 1 a is the base of the instrument as I prefer to construct it. The base
carries two short pillars surmounted by the usual Y's or V shaped notches in which
the axis of the telescope rests. The telescope has two limbs at right angles and an
inclined mirror is fixed at the angle intermediate of the object glass and eye piece.
b is the arm and f the adjusting screw.
The arm bis attached to the vernier plate c. and revolves with it on the axis of
the telescope. d. is the larger divided circle which is fixed to the telescope. When
the instrument is laid in its bearings it has three supports viz, the two ends of the
axis which rest in the Y's and the arm b. which rests against or upon the stand of
the instrument by means of the adjusting screw f. h is a clamping screw for
clamping together the vernier and declination circle. If the divided circle be fixed
in the proper position on the telescope, and if the vernier plate c. be adjusted to the
declination or altitude of any star at transit, that star will be visible in the telescope
when the instrument rests in its bearings with the arm resting against its proper
support at f. In this case there is no need for the declination level hitherto
employed. I term this form of instrument a "window" transit, as it is especially
suited for fixing on a window sill.
Figures 2 and 3 shew a like arrangement to be applied to a transit instrument
of the ordinary form. b, is the arm attached to the veruier plate c, the arm is
shown resting against the adjusting screw f which works in a cock fixed on the
base of the instrument or if preferred on one of the uprights. In Figure 2 the
arrangement is shown in section in order to exhibit more clearly the clamp g. with
its screw fastener h, which clamps the arm b. in any position on the divided circle,
so as to prevent it from shifting round when once set.
Another part of my invention relates to a method of securing the instrument to
its solid foundation whilst providing for adjustment in a convenient manner.
Figure 4 shews this arrangement. The base of the instrument is held securely
down to its pedestal, while free play is allowed to the movements of the levelling
screws. There is no tendency when the holding down screw is tightened to strain
the two uprights of the instrument towards each other. The metal spring i. i.
placed under the base a, is screwed to it at each end by two screws h. h. The base
rests on levelling screws j. j. and is held down with an elastic pressure by the
screw acting on the spring i. i. The screw k passes through a hole in the centre
of the spring and screws into a corresponding hole provided on the top of the pier
or support. Any small adjustment of level is permitted by the yielding of the
spring and may therefore be effected without producing any distortion of the
framework of the instrument.
•
In order to render the support still firmer and steadier, I prefer to use 5 levelling
or supporting screws instead of the number, 3 hitherto employed, In figure 5

page_text: "
"
a. b. and c. are the screws usually employed, and I also use them for the preliminary
adjustment of the instrument-when however it is once fixed and levelled, I bring
down the two additional screws & and e to their bearing and partially or wholly
release the screw c. carefully keeping the level unchanged, the base now rests on 4
screws instead of 3 and is much steadier-while any minute adjustments are
effected as ensily as before, by slightly moving one or both screws on either side.
In the transit instrument it is necessary to set the spider lines or
cross wires
truly central and at right angles to the axis of the instrument in order to avoid
what is known as "collimation error this is usually effected by means of four
straining screws which cause a plate or disc carrying the spider lines to traverse
to and fro. I effect this in a somewhat simpler and cheaper manner-by allowing
the tube of the telescope to pass loosely through the axis or trunnion of the
instrument, and supporting it within the collar of the axis by five or more screws
so placed as to permit of their giving lateral adjustment. This disposition is shown
in figure 6. In this figure a. a, are the top and bottom screws, (one only being seen
in figure 6) and when these only are tightened up, the telescope is free to play
laterally withint he collar-the remaining 4 screws b. b. b. b. are now tightened up
so as to support the tube firmly and at the same time to adjust it so that the centre
wire is in a line with the optical axis of the instrument. Additional screws c, c.
may now be brought into contact if preferred.
The cross hairs of a transit instrument are as before stated usually carried on
a disc which is held and adjusted by four straining screws placed radially and
holding the disc in position by tension. In my improved arrangement
place the
hairs on a second plate or diaphragm which is attached on the end of a short
screwed tube as shewn at m Figure 7 and this tube I screw through the centre of
the disc first above mentioned with a very fine thread by this means and by
screwing round the diaphragm I am enabled to place the hairs vertical with great
facility, and also to adjust their focal distance from the object glass-when the
disposition shewn in Figure 6 is adopted the four straining screws are unnecessary
and the disc becomes a fixture in the tube of the telescope.
The illumination of the cross hairs by night is usually effected by a lantern
shining through a hole in the trunnion of the telescope with a reflecting mirror
in the interior, but sometimes by a small mirror or whitened disc held in front
of the object glass and illuminated by a lantern. I employ in preference to this
a whitened disc of the full size of the object glass, which is hinged on to a cap fitting
on the end of the telescope, and which is perforated in the centre by a hole 1/4 inch
diameter as is shown by Figure 8.
This perforated disc has many conveniences. When quite closed it so contracts
the aperture of the telescope, that a meridan mark may be sharply focussed at a
distance of about 30 yards or less which in many cases is a matter of great con-
venience. When almost closed, but with the whitened face illuminated by the
very oblique light of a lamp showing through the contracted opening, it enables
stars of the first magnitude to be observed with a sharpness and precision not
obtainable by any other disposition, the star, or it may be the sun, being only seen
through the inch aperture. When the whitened face is slightly inclined over
the front of the object glass it enables the light to be graduated to any extent that
may be desired, and, with small instruments gives a kind of illumination which is
superior to that obtained by reflection through the axis. Two hinged discs with
apertures of different sizes may be used.
In order to support the lamp at any elevation so as to suit the varying positions
of the instrument, I insert an upright stem a (see Figure 9) in a socket on the
base of the instrument; this socket carries two pairs of parallel arms b. b. with
clamping screws 6¹ b¹ supporting a tube or socket c. in which the stem of the lamp
rests, an ordinary spring candle lamp being preferred. These two pairs of arms
are both of the same length, and always remain parallel to each other, working
up and down on centres on the upright stem and supporting the lamp socket
vertically at any level by the aid of tightening screws; the arms should be of

page_text: the same length as half the telescope, and, the centre of the upper pair should be
about in a line with the axis of the instrument. The lamp may with advantage
be provided with a bulls eye, and with a hinged reflector on the opposite side for
throwing the light down upon a watch face while observing.
Having thus described the nature of my said Invention and the manner of
performing the same I would have it understood that I claim :-
First. The transit instrument substantially as shewn in Figure 1.
Second The use of an adjustable arm attached to the vernier circle or index,
which arm of a transit instrument rests against a fixed stop on some part of the
frame of the instrument and serves to make the zero point.
Third. The method of connecting a transit instrument to its pin or support
by a spring under base of instrument allowing free play to levelling screws and
avoiding distortion of the frame by undue strain.
Fourth The method of adjusting for collimation by supporting the tube of the
telescope in a loose collar, with adjusting screws for varying its position.
Fiftb. The method of supporting and adjusting the cross wires of a transit
instrument, by means of the screwed diaphragm.
Sixth. The perforated hinged diaphragm with central aperture for illuminating
the cross wires.
Seventh. The jointed lamp support for carrying the lamp.
In witness whereof I the said Josiah Latimer Clark have hereunto set my
hand and seal this Sixteenth day of May in the year of our Lord One
thousand eight hundred and eighty three.
JUSIAH LATIMER CLARK (LS.)