Year: 1898

page_text: Date of Application, 3rd Oct., 1898-Accepted, 18th Feb., 1899
I, NICOLAS FRÉDÉRIC DESIRE BARBIER, of 82, Rue Curial, Paris, France, Manufac-
turer, 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 :-
This invention relates to improvements in projecting mirrors and to improvements
in differential are lamps, as shall now be described reference being had to the
accompanying drawings.
The sphero-elliptical projecting mirror shown in Fig. 1, which may have any
desired "opening," that is to say, may have a short or long focal length, reflects the
light parallel to the axis under better conditions than those of existing systems
of mirrors. This superiority is noticeable principally in the perfect "aplanatism " of
the emerging rays, and furthermore there is no dispersion, because the angles of
incidence and of emergence for each of the rays are practically equal and of opposite
direction.
Let f f be the foci of the ellipse the developement of which is indicated by the
dotted curved line, S O the major axis, F a luminous point, and FA a ray therefrom
which it is desired to deflect in a direction parallel to the axis S f¹, and which meets
the curve E E¹ at the point A. This ray will be refracted along A B and will thus
appear to originate at m. All the refracted rays have their virtual foci on the dia-
caustic M N. These refracted rays meet a sphere having a radius such that after
being reflected they will converge at the point f¹. (This is possible because we may
consider a luminous point situated at f¹ and meeting the sphere R R¹ in such a
manner that, after being reflected, the rays shall not converge in one point, which
cannot take place, but shall constitute practically the catacaustic M N for the sector
under consideration). All the rays emitted from F and comprised in the cone E¹ FE,
have thus been rendered convergent in f¹, after one refraction and one reflection.
Consequently, they must meet anew a separating surface such that they shall become
refracted parallel to the axis. This surface will be an ellipsoid, the eccentricity: (C:CD)
of which will be equal to the inverse of the index of refraction, because if we con-
sider the triangle fCf in which CD is the normal (bisecting the angle of the radii
vectores f C and f¹ C) we have:
Cf
C f¹
2
f D
C f
C f¹
f¹D
or
=
f D
f¹ D
Cf + C f¹
fD + f¹ D;

page_text: This relation which exists for all the points of the mirror, proves the "aplanatism
of the mirror.
The projecting mirror may also be composed of annular elements of the form
shown in Figs. 2, 3 and 4, which elements are analogous to those already employed
in the optical apparatus of light-houses, but which would be silvered on the external
face, indicated by the short arrows. In certain cases, these mirrors may offer advan-
tages by reason of their mode of construction, which allows of obtaining great
accuracy, and because it is possible to put together the elements which constitute
these mirrors, edge to edge, in such a manner as to form a plane or curved internal
surface of regular form, or, keeping them in place by means of toothed racks and
setting them back, whilst maintaining a certain space between them with the object
of improving the ventilating conditions, and also the conditions for resisting shock
and jolting produced by firing guns.
The mirror shown in Fig. 2 somewhat resembles a Fresnel lens which would be
silvered on its external surface, and it may be calculated in the same manner but by
substituting for the refraction of emergence, a reflection, that is to say, a refraction
the deviation of which is given by employing the index 1. In this mirror, the re-
fraction on emergence is nil, as the rays emitted from the focus undergo, on entering
the mirror, one refraction, then reach the silvered face and are reflected; the rays
being then directed toward the non-silvered face and leave the mirror without re-
fraction, because they are then at right angles to the outlet face.
Fig. 3 illustrates an arrangement of elements in which the refraction of the incident
rays is nil. The rays have the same direction as those in Fig. 2 but have no refraction
on entering, as they are directed at right angles to the surface of inlet whilst they
have one refraction at the outlet surface. In the two mirrors shown in Figs. 2 and 3
a slight dispersion is produced at the edges.
The type of mirror shown in Fig. 4 and which may be called an equi-angular
mirror, has a form in cross section intermediate between those shown in Figs. 2
The angles of incidence and of emergence for one and the same ray are
equal. In this arrangement, as in a prism of total reflection, the dispersion
is nil.
and 3.
In this kind of mirror the direction of the rays is likewise the same as the fore-
going, but they undergo two equal refractions one on entering and one on leaving
the mirror, and one reflection upon the silvered face.
The cross sectional form of the mirror shown in Fig. 5 shows that the mirrors
according to the cross sectional forms shown in Figs. 3 and 4 may be combined with
catadioptric rings of any kind in such a manner as to project luminous pencils of
rays composed of rays comprised within a sector extending up to about 270°. In
this figure is shown a mirror, the outer face of which is silvered, (and which is of a
type similar to the mirror shown in Fig. 3) having three catadioptric elements, i.e.
elements calculated so that the rays entering the prism with a first refraction
reach the outer face under such an angle of incidence that they are reflected
thereupon without its necessarily being silvered. After reflection, these rays go
through the prism and leave with a fresh refraction. The central portion of the
mirror having rings which are silvered on their outer face may be according to either
one of the forms illustrated in Figures 3 and 4.
In a differential arc lamp according to this invention, the device for moving
the carbons towards each other or away from each other, comprises a small electro-
motor, operating in the manner now to be described by the aid of Figs. 6, 7 and 8 of
the accompanying drawings.

page_text: The field magnets a are constantly excited in a shunt circuit from the terminals bb
of the lamp, whilst the armature is provided with two distinct windings c, c¹ which
allow the motor to rotate in one or the other direction, according as a shunt of
the main current is sent in one or the other of these two windings.
The distribution of the current to one of these windings of the arinature is
effected by means of a commutator i, j, j operated by a relay. This relay is
provided with two bobbins, one, d, in series with the lamp and traversed by the
main current. The other being in shunt from the terminals of the lamp.
These two bobbins may be wound upon the same core if desired.
These two bobbins produce in the normal working of the lamp two equal fields
in opposition to each other. The two windings a and d¹ instead of forming two
distinct shunt circuits may be placed in series and may form only one shunt
circuit, from the terminals bb of the lamp. 1 is an insulator serving to separate
electrically the part ff from the contact which permits of short circuiting the
armatures.
If for any reason, wear of the carbons e e¹ or lighting the lamp, for example,
these two fields are no longer equal, the part ff in the form of an inverted "T"
mounted and balanced on a point g, inclines towards that side where the field is more
intense, (see Fig. 7) then the distributor i, being operated by the pallet, bears against
the contact and sends the current into the armature c, thereby producing a
suitable shifting of the carbon e. A short circuit of the armature produced by
contact lensures the immediate stoppage of the motor as soon as the regulation has
been effected, and the two fields of the relay are equal. The pallet is then maintained
in this position by means of two coiled springs has shown, or the latter may if
desired be replaced by a single leaf spring.
Each carbon holder ¹ is mounted at the ends of two rods k, l, (shown in Fig. 8)
which are so arranged that they are always parallel and corresponding in position.
For that purpose the rods are adapted to slide with slight friction in two guiding
sockets m, ml each pivoted on an axle n, n¹; which sockets m, m¹ may, if desired,
be situated at any desired part of the rods, . Furthermore, the two rods k, kt
carry two guides o, o¹ each sliding in a groove p, p¹. These two guides and slides
may, if desired, be replaced by cranks.
In imparting to one of the axles n, n¹ rotary movement by means of the small
motor of the lamp, the parallelogram n, n¹, 12, 12, is distorted, and the slides p, p¹ or
the cranks which replace them, are arranged in such a manner that the carbon holder
fixed to the upper part of the rods will describe a straight line corresponding to
the cominon axis of the two carbons e, e¹. Any ordinary device allows, if necessary,
of stopping the automatic moving together of the carbons and of effecting the said
movement by hand.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed, I declare that what
I claim is
1. Sphero-elliptical mirrors constructed substantially as herein before described.
2. Lens shaped mirrors having a plane, curved or equi-angular face, as herein
described, and the outer surface of which is silvered, whether the elements con-
stituting the said face are jointed together flush with one another, or are set back
relatively to one another, as set forth.
3. The combination of the said mirrors with catadioptric elements.
4. In an electric arc lamp the use of an electric motor having a constantly-excited
field magnet a and an armature provided with two distinct windings c c¹, the current
being distributed to the said windings by means of a relay comprising two
bobbins & done of which is arranged in series with and the other as a shunt, to the
carbons, whether or not the said bobbins be wound upon the same core.
5. In an are lamp, the application, to the regulation of carbon holders, of a jointed
parallelogram, substantially as herein described with reference to and shown in Fig. 8
of the drawings annexed.

page_text: 6. Means for regulating the arc of electric are lamps constructed arranged and
operating substantially as hereinbefore described with reference to and shown in
Figs. 6 and 7 of the accompanying drawings.
Dated this 3rd day of October 1898.
F. WISE HOWORTH,
46, Lincoln's Inn Fields, London, W.C., Agent for the Applicant.