IMPROVEMENTS IN OBJECT GLASSES FOR ASTRONOMICAL TELESCOPES AND OTHER OPTICAL INSTRU- MENTS
Year: 1878

page_text: LETTERS PATENT to Alexander Melville Clark, of 53, Chancery Lane, in the
County of Middlesex, Patent Agent, for the Invention of "IMPROVEMENTS IN
OBJECT GLASSES FOR ASTRONOMICAL TELESCOPES AND OTHER OPTICAL INSTRU-
MENTS."
A communication from abroad by Ernst Gundlach and John Jacob
Bausch, both of Rochester, and Henry Lomb, of Brooklyn, both in the State
of New York, United States of America.
Sealed the 24th May 1878, and dated the 15th February 1878.
PROVISIONAL SPECIFICATION left by the said Alexander Melville Clark
at the Office of the Commissioners of Patents on the 15th February 1878.
ALEXANDER MELVILLE CLARK, of 53, Chancery Lane, in the County of Mid-
dlesex, Patent Agent. "IMPROVEMENTS IN OBJECT GLASSES FOR ASTRONOMICAL
TELESCOPES AND OTHER OPTICAL INSTRUMENTS." [A communication from abroad by
Ernst Gundlach and John Jacob Bausch, both of Rochester, and Henry Lomb, of
Brooklyn, both in the State of New York, United States of America.]
The object of this Invention is to construct object glasses for astronomical
telescopes and for other optical purposes, in which the aberrations, spherical as well
as chromatic, are corrected to a higher degree than has been the case in any
objective hitherto constructed.
The common double objective hitherto used, and consisting of a negative flint
glass lens and positive crown glass lens, is deficient by reason of chromatic over-
correction at the outer edge, and chromatic under-correction toward the centre.
This is caused by the flint glass lens as usually shaped in these objectives not having
the proper form to remove this defect. Nor can it be perfectly removed by any
alteration in the shape of the same, except at the expense of increased spherical
aberration, the correction of both aberrations depending on the same factor (the flint
glass lens) and on opposite conditions of this factor, the best form for the complete
correction of the one producing the maximum of the other aberration.

page_text: This difficulty is obviated, according to this Invention, by constructing an
objective in which both the chromatic and the spherical aberrations are corrected
by special means, independent of each other, leaving the flint glass lens to perform
exclusively its legitimate and original function of correcting the chromatic aberra-
tion, without reference to the spherical aberration, and correcting the latter by one
or more negative crown glass lenses of the proper focal relations to the other lenses
composing the objective.
According to this Invention I construct the objective with a flint glass lens,
which is concave on both sides, instead of the convex-concave flint glass lens ordi-
narily used in double objectives. This double concave form is well known and
admitted to be the best form for the highest possible correction of the chromatic
aberration, the concavo-convex form used in the double objective having been
adopted only as a compromise, aiming at the correction of both aberrations at the
same time, which, for the reasons 1 have above stated, can be only imperfectly
attained under such circumstances. This doubly concave flint glass lens I place
between two positive crown glass lenses of appropriate focal proportions. By this
means the objective can be made almost absolutely achromatic, leaving the spherical
aberration to be corrected by other and independent means.
The correction of the spherical aberration I effect by means of a special negative
crown glass lens (or if preferred, more than one negative crown glass lens may be
employed) concavo-convex in shape, the concave surface of which has a shorter
radius than the convex surface of the positive crown glass lens next to it, so that
between the two a space remains in the shape of a meniscus. In this way the
correction of one aberration is made independent of the other, and the correction of
both may be effected to a higher degree of perfection.
The loss of illuminating power on account of the increased number of surfaces can
hardly be considered a valid objection against this form of an objective, as said loss
can be reduced to a minimum by cementing by means of a proper medium the
adjoining surfaces, said loss being further reduced in comparison with the double
objective by having in the above described objective the outer surfaces consisting
of crown glass, the loss of light on such surfaces being less than on flint glass
surfaces. It will, moreover, be preponderatingly compensated by the better cor-
rection of the aberrations, and the greater clearness and sharpness of definition
resulting therefrom.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Alexander Melville Clark in the Great Seal Patent Office on the
14th August 1878.
ALEXANDER MELVILLE CLARK, of 53, Chancery Lane, in the County of Middlesex,
Patent Agent. "IMPROVEMENTS IN OBJECT GLASSES FOR ASTRONOMICAL TELESCOPES
AND OTHER OPTICAL INSTRUMENTS." A communication from abroad by Ernst
Gundlach and John Jacob Bausch, both of Rochester, and Henry Lomb, of
Brooklyn, both in the State of New York, United States of America.
The object of this Invention is to construct object glasses for astronomical
telescopes and for other optical purposes, in which the aberrations, spherical as well
as chromatic, are corrected to a higher degree than has been the case in any objective
hitherto constructed.
The common double objective hitherto used, and consisting of a negative flint
glass lens and positive crown glass lens, is deficient by reason of chromatic over-
correction at the outer edge and chromatic under-correction toward the centre.
This is caused by the flint glass lens, as usually shaped in these objectives, not
having the proper form to remove this defect. Nor can it be perfectly removed by
any alteration in the shape of the same, except at the expense of increased spherical
aberration, the correction of both aberrations depending on the same factor (the
flint glass lens) and on opposite conditions of this factor, the best form for the com-
plete correction of the one producing the maximum of the other aberration.
According to this Invention this difficulty is obviated by constructing an object
glass, in which both the chromatic and the spherical aberration are corrected by
special means independent of each other, leaving the flint glass lens to perform
exclusively its legitimate and original function of correcting the chromatic aberra-
tion without reference to the spherical aberration, and correcting the latter by one
or more negative crown glass lenses of the proper focal relations to the other lenses
composing the objective.
The accompanying Drawing represents a cross section of an object glass con-
structed in accordance with this Invention, A representing the flint glass lens, B
and C crown glass lenses, and D an additional negative crown glass lens, the purpose
of this latter being the completest correction possible of the spherical aberration.
As will be readily seen from the figure, the objective glass is constructed with a
flint glass lens A, which is concave on both sides instead of the convex-concave flint
glass lens ordinarily used in double objectives. This double concave form is well
known, and admitted to be the best form for the highest possible correction of the
chromatic aberration, the concavo-convex form used in the double objective having
been adopted only as a compromise aiming at the correction of both aberrations at
the same time, which for the reasons above stated can be only imperfectly attained
under such circumstances. This doubly concave flint glass lens A is placed between
two positive crown glass lenses B and C, of appropriate focal proportions. By this
means the objective can be made almost absolutely achromatic, leaving the spherical
aberration to be corrected by other and independent means.
The correction of the spherical aberration is effected by means of a special nega-
tive crown glass lens D (or, if preferred, more than one negative crown glass lens
may be employed), concavo-convex in shape, the concave surface of which has a
shorter radius than the convex surface of the positive crown glass lens next to it,
so that between the two a space remains in the shape of a meniscus.
In this way
the correction of one aberration is made independent of the other, and the correction
of both may be effected to a higher degree of perfection.
The loss of illuminating power on account of the increased number of surfaces
can hardly be considered a valid objection against this form of object glass, as said
loss can be reduced to a minimum by cementing by means of a proper medium
the adjoining surfaces, said loss being further reduced in comparison with a double
object glass by having in the above-described object glass the outer surfaces con-

page_text: sisting of crown glass, the loss of light on such surfaces being less than on flint glass
surfaces. It will, moreover, be preponderatingly compensated by the better cor-
rection of the aberrations and the greater clearness and sharpness of definition
resulting therefrom.
Having described the nature of the said Invention, and the manner of performing
the same, I declare that what I claim as the Invention to be protected by the
herein-before in part recited Letters Patent, is,-
1st. An object glass for telescopes and other optical instruments, wherein the
spherical aberration is corrected independent of the chromatic aberration by means
of one or more separate negative (concavo-convex) lenses, the whole constructed in
the manner and for the purposes substantially as described.
2nd. One or more negative crown glass lenses, in combination with positive crown
glass and negative flint glass lenses, substantially in the manner and for the purpose
herein specified.
In witness whereof, I, the said Alexander Melville Clark, have hereunto set
my hand and seal, this Fourteenth day of August, in the year of our Lord
One thousand eight hundred and seventy-eight.