A New Catadioptric Projector.
Year: 1899

page_text: A New Catadioptric Projector.
I, JOHN EBENEZER BOUSFIELD, of the Firm of G. F. Redfern & Co., The
General Patent Office, No. 4, South Street, Finsbury, in the County of London,
Patent Agents, do hereby declare the nature of this invention (a communica-
tion from Angelo Salmoiraghi, of Milan, in the Kingdom of Italy, Engineer)
to be as follows:-
:-
This invention relates to a catadioptric projector consisting of a large sphero-
parabolic concave mirror and of a smaller convex mirror. These two mirrors
are in the same axial line and are provided in the middle with apertures in
front of which are placed convergent lenses.
The source of light is placed outside in such a manner that the rays which
emanate therefrom are directed at first through the lens situated in front of the
aperture in the large mirror towards the convex surface of the small mirror
which reflects them and directs them on to the concave surface of the large
mirror whence they issue in the form of a cluster of parallel rays.
The rays which pass through the first lens in proximity to the principal axis
would be reflected by the convex mirror towards the source of light if a hole
were not made in the centre of the mirror. The result is that all the rays
situated near the axis pass through this hole after having been directed
parallelwise by the small lens.
The glass wall which forms the front surface of the projector is of lenticular
shape and this shape is so chosen that in employing spherical mirrors any dis-
persion of the light may be avoided.
The diameter of each aperture is about one fifth of that of the mirror.
The focal distance of the small lens should be calculated so that the virtual
image of the source of light shall coincide with the focus of the large mirror.
The large lens thus first forms a real image of the source of light; the small
convex mirror produces a virtual image of the said source; as this point is also
the focus of the convex mirror, the luminous rays issue in a direction parallel
with the principal axis.
The rays proceeding from the source of light situated sufficiently near to the
axis to be reflected by the small mirror on to the large lens are collected by the
small lens and issue through the hole in the convex mirror parallel with the axis.
The focus of the small lens should consequently coincide with the focus of the
large lens. The dimensions of the apparatus, the diameters and the focal dis-
tances of the lenses are calculated so as to theoretically avoid any loss of light.
Without regarding the inevitable absorption resulting from the material con-
struction of the apparatus, it may be said that all the luminous rays which fall
from the source of light on to the large lens issue from the apparatus in a parallel
direction.
The described combination of mirrors and lenses is fitted in a projector
cylinder. A glass disc closes in the apparatus. If the small mirror is not para-
bolic but consists simply of a spherical mirror which is much more easy to con-

page_text: struct, the said glass disc will take the form of a convex lens so as to neutralize
the deviation arising from the spherical shape of the mirror and to permit the
rays to issue from the apparatus in a parallel direction. The described arrange-
ment possesses several important advantages.
1. All the parts of the optical apparatus can be mounted within a closed
cylinder, reducing the dimensions and weight of the said parts and protecting
them from all causes of deterioration, whilst a lateral aperture will allow of
cleaning them without extinguishing the lamp.
2. The parts of the optical apparatus are no longer exposed to the high
temperature of the lamp, which is placed outside.
3. Mirrors with limited angular apertures can be employed, thereby obviating
the difficulties in the construction of the parabolic surfaces.
4. Several projectors can be used with a single luminous source by placing
the latter at the meeting point of the axes of the projectors. This very important
property opens a new way to the construction of maritime pharos and of inter-
mittent apparatuses, it being possible to mount the different projectors having á
common focus upon a single support.
Datad
5141 Jaw of .... 1000
G. F. REDFERN & Co.,
4, South Street, Finsbury, London,
Agents for the Applicant.
A New Catadioptric Projector.
I, JOHN EBENEZER BOUSFIELD, of the Firm of G. F. Redfern & Co., The
General Patent Office, No. 4, South Street, Finsbury, in the County of London,
Patent Agents, do hereby declare the nature of this invention (a communica-
tion from Angelo Salmoiraghi, of Milan, in the Kingdom of Italy, Engineer),
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 a catadioptric projector consisting of a large sphero-
parabolic concave mirror and of a smaller convex mirror. These two mirrors
are in the same axial line and are provided in the middle with apertures in
front of which are placed convergent lenses.
The source of light is placed outside in such a manner that the rays which
emanate therefrom are directed at first through the lens situated in front of the
aperture in the large mirror towards the convex surface of the small mirror
which reflects them and directs them on to the concave surface of the large
mirror whence they issue in the form of a cluster of parallel rays.
The rays which pass through the first lens in proximity to the principal axis
would be reflected by the convex mirror towards the source of light if a hole
were not made in the centre of the mirror. The result is that all the rays
situated near the axis pass through this hole after having been directed
parallelwise by the small lens.
The glass wall which forms the front surface of the projector is of lenticular
shape and this shape is so chosen that in employing spherical mirrors any dis-
persion of the light may be avoided.
In the accompanying drawings,
Figure 1 illustrates diagrammatically the direction of the luminous rays
through a projector of this kind;

page_text: Figure 2 is a longitudinal section of the projector taken through the cylinder.
Figure 3 is a front view of the same.
,y (Figure 1) is the principal axis of this apparatus which is also that of
the concave mirror S, S and of the convex mirror s, s each of the two mirrors
having a circular hole in the middle. The diameter of each hole is about one
fifth of that of the mirror. In front of each hole is placed a convergent lens
(C, C, and c, c respectively). The luminous rays proceeding from the point L
are concentrated in the focus L¹ of the lens C, C, and fall on to the convex
mirrors, which directs them towards the concave surface of the large
mirror S, S which in its turn, reflects them in a direction parallel with that of
the principal axis. The focal distance of the lens s, s should be calculated so
that the virtual image of the point L' shall coincide with the focus F of the
mirror S, S. The lens C, C thus first forms a real image L of the source of
light L the convex mirrors, s produces a virtual image of the said source at F;
as this point is also the focus of the convex mirror, the luminous rays issue
in a direction parallel with the principal axis.
The rays proceeding from the source of light L situated sufficiently near to
the axis ili, y to be reflected by the mirror 8,
on to the
lens C, C are collected by the small lens c, and issue through the hole in the
convex mirror s parallel with the axis. The focus of the small lens c, c
should consequently coincide with the focus L' of the lens C, C. The dimensions
of the apparatus, the diameters and the focal distances of the lenses are calcu-
lated so as to theoretically avoid any loss of light. Without regarding the in-
evitable absorption resulting from the material construction of the apparatus,
it may be said that all the luminous rays which fall from the source of light L
on to the lens C, C, issue from the apparatus in a parallel direction.
Figures 2 and 3 represent the described combination of mirrors and lenses
fitted in a projector cylinder. The lens c is placed behind the convex
mirrors, s instead of being placed in front of the same; but that will cause no
alteration in the operation. a indicates a glass disc which closes in the apparatus.
If the mirrors, s is not parabolic but consists simply of a spherical mirror which
is much more easy to construct, the glass disea will take the form of a convex
lens so as to neutralize the deviation arising from the spherical shape of the
mirror and to permit the rays to issue from the apparatus in a parallel direction.
The described arrangement possesses several important advantages, namely,―
1. All the parts of the optical apparatus can be mounted within a closed
cylinder, reducing the dimensions and weight of the said parts and protecting
them from all causes of deterioration, whilst a lateral aperture will allow of
cleaning them without extinguishing the lamp.
2. The parts of the optical apparatus are no longer exposed to the high tem-
perature of the lamp, which is placed outside.
3. Mirrors with limited angular apertures can be employed, thereby obviating
the difficulties in the construction of the parabolic surfaces.
4. Several projectors can be used with a single luminous source by placing
the latter at the meeting point of the axes of the projectors. This very important
property opens a new way to the construction of maritime pharos and of inter-
mittent apparatuses, it being possible to mount the different projectors having
a common focus upon a single support.
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. In a catadioptric projector, the arrangement of a larger concave mirror
(S, S) and a small convex mirror arranged axially therewith, both mirrors
being provided with central holes and with convergent lenses in such a manner
that the light rays issuing from the source of light arranged outside the appara-
tus are converged by the lenses (C, C) and are projected upon the convex

page_text: mirror (s, s) from which they are directed to the concave mirror (S, S) and are
directed by the concave mirror as a cluster of rays parallel to the axis , Y,
and issue through the opening of the smaller mirror (s, s) in a parallel manner
for the purpose of causing the cluster of rays which fall upon the lens (C, C) to
issue in a parallel manner from the apparatus, substantially as described.
2. The arrangement of the lenses and mirrors described in Claim 1 in the
cylinder of a projector, the front glass a of which is arranged as a convergent
lens in order when spherical mirrors are employed to obviate the dispersion of
the light and to direct the rays parallel, substantially as set forth.
3. The manufacture and use of catadioptric projectors constructed, arranged
and operating substantially in the manner hereinbefore described and illustrated
in the accompanying drawings.
TL
011
1000
G. F. REDFERN & Co.,
4, South Street, Finsbury, London,
Agents for the Applicant.