IMPROVEMENTS IN OBTAINING IMAGES ON MÉTALLIC AND OTHER SURFACES
Year: 1842

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, RICHARD
HODGSON, of Montague Place, in the County of Middlesex, Gentleman, send
greeting.
WHEREAS Her present most Excellent Majesty Queen Victoria, by Her
Letters Patent under the Great Seal of Great Britain, bearing date at West-
minster, the Seventh day of July, in the sixth year of Her reign, did, for
Herself, Her heirs and successors, give and grant unto me, the said Richard
Hodgson, Her especial license, full power, sole privilège and authority, that I,
the said Richard Hodgson, my exors, adñiors, and assigns, or such others
as I, the said Richard Hodgson, my exors, admors, or assigns, should at
any time agree with, and no others, from time to time and at all times
during the term of years therein expressed; should and lawfully might make,
use, exercise, and vend, within England, Wales, and the Town of Berwick-
upon-Tweed, my Invention of "IMPROVEMENTS IN OBTAINING IMAGES ON MÉTALLIC
AND OTHER SURFACES;" in which said Letters Patent is contained a proviso
that I, the said Richard Hodgson; shall cause a particular description of the
nature of my said Invention, and in what manner the same is to be performed,
to be inrolled in Her said Majesty's High Court of Chancery within six calendar
months next and immediately after the date of the said in part recited
Letters Patent, as, in and by the same, reférence being thereunto had,' will
more fully and at large appear.
NOW KNOW YE, that in compliance with the' said proviso, I, the' said
Richard Hodgson, do hereby declare that the nature of the said Invention,
and the manner in which the same is to be performed, are fully described and.

page_text: ascertained in and by the following statement thereof, reference being had to
the Drawing hereunto annexed, and to the figures and letters marked
thereon (that is to say):-
My Invention relates, first, to improvements in that class of optical instru-
ments where images are obtained on surfaces by reflecting mirrors, and
consists in causing the images so reflected to pass through a lens or lenses,
interposed between the mirror and the receiving surface; and it also consists
of passing the image from a mirror through a tube or trunk interposed between
the mirror and the receiving surface.
And, secondly, my Invention relates to improvements in refracting cameras,
by applying a lens or lenses, as hereafter explained. And in order that the
Invention may be most fully described and readily understood, I will describe
the Diagrams hereunto annexed.
Figure 1 is the camera box of twelve inches long, containing the mirror
ground of different concavities, the first surface being of ten inches radius, the
second or silvered surface of sixteen inches radius. The object is supposed to
be the sun or a star, at an infinite distance, the rays from which are parallel
before reflection, and converge afterwards to the line D; but by interposing.
an achromatic lens B in a tube B*, the rays E converge to a focus on the
point C, and in passing through the achromatic lens not only is much of
the spherical aberration corrected, but also the chromatic suffers a great
diminution.
Figure 2: the mirror A is placed in a similar box to Figure 1; the mirror
is also of the same radii. The object is supposed to be at a near distance, say
eight or ten feet. The dotted lines E are rays from the extreme points of the
object, which after reflection would form a curved image of the object on the
line D, but by interposing the correcting lens (in this instance a meniscus of
fourteen inches focus), the convergence of the rays is increased, and much of
the spherical aberration being done away with, the image of the object is
formed upon the line C. In the diminution of the spherical aberration much
also of the chromatic is corrected. The advantages which I consider I have
gained by the introduction of a correcting lens are, first, the ability to use a
mirror of any dimensions, whereas in the present reflecting cameras the apertures
are all obliged to be diminished, by the insertion of diaphragms to cut off the
aberrations. Second, by increasing the aperture I have an increase of light
in proportion to the increase of the square of the surface of the mirror.
Third, the correcting lens having concentrated the power of the light, enables
me to operate in less time than without it. Fourth, the receiving surface
being enclosed in a tube or box within the camera, enables me to have

page_text: the mirror much nearer the front of the camera. The images thrown by a
concave reflector are convex. The images thrown by a lens (either meniscus
or compound achromatic) are concave, and by the union of the two the errors
of both are to a great extent compensated for; and I believe wherever the
spherical aberration is diminished by passing through two (media) glasses,
much of the chromatic is corrected. It will therefore be seen that the
Invention partly consists of the introduction of a correcting lens or lenses
(either simple or compound) between the mirror and the receiving surface,
for the purpose of abating both chromatic and spherical errors, such lens or
lenses for photographic purposes to be inclosed in a tube, with the power of
adjusting according to the distance of the objects. For astronomical or
telescopic purposes I add another lens or lenses, after the image has reached
its focus, for the purpose of magnifying or increasing it, and then put the
receiving surface further back. I do not confine myself to mirrors of unequal
radii, as I can apply mirrors with parallel surfaces, and also metallic mirrors
with only one surface; neither do I confine myself to using the tube or trunk
when for photographic purposes, with a lens; for I find, even without a lens,
tbe tube has a decided superiority over the diaphragms now in use.
Figure 3 is a tube for a refracting camera, consisting of a prism A and B
of different glasses, the first A being a double convex lens of crown glass,
forming with the prism B, of flint glass, an achromatic object glass, the rays
from which would form a focus at eight inches behind them. The lenses
D and E form a correcting lens. D is a concave convex flint glass, with its
convex side towards the object. E is a double convex crown glass of unequal
radii, the lesser radius being towards the image. This correcting lens of
twelve inches focus increasing the concentration of the rays, they will form
the image about four inches behind it, increasing at the same time its
intensity. C is a tube holding the prism sliding within another tube containing
the lens D, E, to enable an adjustment according to the distance of the
object.
The advantages gained by this combination are, first, the erecting of the
image, which is of great importance, particularly in portraiture, for by the
optical arrangements at present in use in refracting cameras, the right hand
or right eye of the sitter is inverted, and caused to represent the left. This,
I am aware, has been corrected by introducing a flat reflector, which occasions
great loss of light, and a prism of flat surfaces has also been placed before the
lenses. But the improvement consists in the introduction of a prism with
curved or worked surfaces, such prisms being either single or compound,
with silvered surfaces or unsilvered; and also for the introduction of a lens or

page_text: lenses placed behind the prism, to correct and gather the rays to a shorter
focus. I do not attempt to state the quantity of light lost by passing through
the prism, but I can obtain images from the living figure in the shade by this
optical arrangement in a very short time. Second advantage is in the-tubes,
by which I can get a more perfect image of an object at different distances
than by the method at present practised, of fixing the glasses in the same
tube. The image is also much sharper and more defined. I do not confine
myself to passing the rays through the prism first, for I find I can obtain a
similar result by passing the image through the lens first, and correcting the
rays by passing them through the prism afterwards.
. Having thus described the nature of my Invention, and the manner of
performing the same, I would have it understood that what I claim is,―
First, the mode of constructing optical instruments (where images are obtained
on to surfaces by reflecting mirrors), in such manner that the images reflected
are caused to pass through a lens or lenses, interposed between the mirror
and the receiving surface. And I also claim the passing of the image or
images from a mirror through a tube interposed between the mirror and the
receiving surface.
And, secondly, I claim the mode of combining a prism of curved surfaces
with a lens or lenses in refracting cameras, as above described.
In witness whereof, I, the said Richard Hodgson, have hereunto set my
band and seal, this Seventh day of January, in the year of our Lord
One thousand eight hundred and forty-three.

page_text: fall Zize
D
A Mirror of Glass of different Curvatures the first Face 10 Inch radius the Silvere
surface 16 radius.
B. AMeniscus of 14 Inch Focus.
B. The Sliding tube containing the lens.
C. The Surface at focus after passing through the lens.
D. The focus without the correcting lens.
E. Rays from the Mirror before passing through the lens.
E. Rays shown after passing through the lens.
F. Small stop or Cell to hold the lens.
Note The dotted lines Exshown in the diagram are intended generally to indicate
the supposed position of the rays but they are not intended to represent those
rays with mathematical correctness.
1. Mirror of Glass of different Curvatures the First face
10 Inch radius.
The Silvered surface 16 Inch radius
3. An achromalic lens of & Inch focus with the Flint placed
towards the Mirror.
3 The tube containing the dens.
C. The point of Focus.
. The point of focus of the Mirror without the correcting lens.
E. Rays from the Mirror before passing through the lens.
Rays shown after passing through the lens supposing th
object to be the Sun or a Star at an infinite distance.
ote.The dotted Rays Eshown in the diagram are intended
generally to indicate the supposed positions of the rays
but they are not intended to represent those Rays with
Htical Correctness.