IMPROVEMENTS IN OPERA GLASSES, FIELD GLASSES, AND OTHER GALILEAN TELESCOPES
Year: 1870

page_text: LETTERS PATENT to John Henry Dallmeyer, of No. 19, Bloomsbury
Street, in the County of Middlesex, Optician, for the Invention
of "IMPROVEMENTS IN OPERA GLASSES, FIELD GLASSES, AND OTHER GALILEAN
TELESCOPES."
Sealed the 21st March 1871, and dated the 19th October 1870.
PROVISIONAL SPECIFICATION left by the said John Henry Dallmeyer
at the Office of the Commissioners of Patents, with his Petition,
on the 19th October 1870.
I, JOHN HENRY DALLMEYER, of 19, Bloomsbury Street, in the
County of Middlesex, Optician, do hereby declare the nature of the
said Invention for "IMPROVEMENTS IN OPERA GLASSES, FIELD GLASSES, AND
OTHER GALILEAN TELESCOPES," to be as follows:-
This Invention has for its object improvements in opera glasses, field
glasses, and other Galilean telescopes, and consists in the construction of
such telescopes with double combination cemented object glasses in
which the denser refracting medium, as the flint glass, occupies the
i
.

page_text: external position, i.c., is exposed to the object to be viewed or looked
at, while the lesser refracting medium, as the crown glass, is nearest to
or faces the eye lens or lenses.
The external or flint glass lens is a concavo-convex, convex side outside
or external, and the crown glass lens is by preference a convexo-convex
or plaino-convex lens, the adjacent surfaces of these two lenses, i.c., the
concave of flint and the convex of crown are identical and united by a
permanently transparent cement. The ratio of radii of the external and
internal surfaces are so apportioned that for the kinds of glass employed
in the construction of the object glass, composed of lenses of the above
form and position, the spherical as well as the chromatic aberrations of the
central as well as of the oblique pencils are perfectly corrected, while at
the same time there is no perceptible distortion even for a large extent of
field of view. Two points not simultaneously or conjointly attained in any
of the existing forms of Galilean telescopes the defects in existing glasses
being perceptible by a haze or defective definition always observable in
the centre of the field, as well as imperfect definition at the margins owing
to a certain amount of residual spherical aberration left uncorrected or
distortion of objects is apparent when viewed at the marginal parts of the
field, i.e., straight lines appear curved and generally convex towards
the centre of the field. Either or both of these defects augment in
proportion as the magnifying power is increased, or as the field of
view is enlarged, and owing to these defects the Galilean telescope has
usually been limited to, or constructed only of comparatively low
magnifying power, i.e., for viewing objects at short distances, and these
drawbacks no longer exist in the new form of object glass as described.
In some cases a Barlow lens or a Roger's lens is interposed between
the object glass and the eye glass.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said John Henry Dallmeyer in the Great Seal Patent 30
Office on the 19th April 1871.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JOHN
HENRY DALLMEYER, of 19, Bloomsbury Street, the County of Middlesex,
Optician, send greeting.

page_text: WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Nineteenth day of October, in the year
of our Lord One thousand eight hundred and seventy, in the thirty-
fourth year of Her reign, did, for Herself, Her heirs and successors,
give and grant unto me, the said John Henry Dallmeyer, Her special
licence that I, the said John Henry Dallmeyer, my executors, adminis-
trators, and assigns, or such others as I, the said John Henry Dall-
meyer, my executors, administrators, and assigns, should at any time
agree with, and no others, from time to time and at all times thereafter
during the term therein expressed, should and lawfully might make,
use, exercise, and vend, within the United Kingdom of Great Britain
and Ireland, the Channel Islands, and Isle of Man, an Invention for
"IMPROVEMENTS IN OPERA GLASSES, FIELD GLASSES, AND OTHER GALILEAN TELE-
SCOPES," upon the condition (amongst others) that I, the said John
Henry Dallmeyer, my executors or administrators, by an instrument
in writing under my, or their, or one of their hands and seals, should
particularly describe and ascertain the nature of the said Invention,
and in what manner the same was to be performed, and cause the
same to be filed in the Great Seal Patent Office within six calendar
months next and immediately after the date of the said Letters

page_text: telescopes with double combination cemented object glasses, in which
the denser refracting medium is external.
In witness whereof, I, the said John Henry Dallmeyer, have
hereunto set my hand and seal, this Eighteenth day of April,
in the year of our Lord One thousand eight hundred and seventy-
one.