Improvements in Devices for Observing or Reproducing the Images Projected by Optical Lens Systems.
Year: 1899

page_text: Date of Application, 17th July, 1899
Complete Specification Left, 10th Apr., 1900-Accepted, 12th May, 1900
Improvements in Devices for Observing or Reproducing the Images
Projected by Optical Lens Systems.
I, Professor ERNST ABBE, Doctor of Philosophy, of Carl-Zeiss-Strasse at Jena,
Grand-duchy of Saxe-Weimar, Germany, do hereby declare the nature of this inven-
tion to be as follows:-
Where a lateral part of an image produced by an optical lens system is to be
observed or reproduced by means of a second lens system, according to the present
invention the said lateral part is (really or virtually) turned by the intervention of a
prism so as to stand perpendicularly to the pencils of rays which-(really or virtually)
issuing from the so turned image-traverse the said second lens system. This prism
is so located, that its principal section contains the axis of the first lens system, and
it may be either a deflecting prism or a reflecting one of non-isosceles section.
Improvements in Devices for Observing or Reproducing the Images
Projected by Optical Lens Systems.
I, ERNST ABBE, Doctor of Philosophy, of Carl-Zeiss-Strasse, Jena, Grand-Duchy of
Saxe-Weimar, German Empire, 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:―
The invention relates to optical devices for observing or reproducing the image
projected by a lens system and it consists in a supplemental means for compassing or
facilitating with such devices correct observation or reproduction of lateral parts of the
image, that is to say parts which are less or more distant from the axis of the system.
In the annexed drawing:
Fig. 1 is a diagram showing the difficulty of observing or reproducing lateral parts
of an image projected by a lens system.
Fig. 2 is another diagram showing how the said difficulty is overcome by the
interposition of a deflecting prism.
Fig. 3 is a third diagram showing an arrangement, in which a non-isosceles
reflecting prism is substituted for the said deflecting prism.
In operations with optical instruments it is sometimes desirable to reproduce, by
means of an additional lens system, an eccentric part of the image projected by a lens
system. In the example shown in Fig. 1 the task is to control the correctional
DQ27

page_text: condition of the photographic objective A as to oblique pencils of rays, that is to say
to examine the quality of the peripheric parts of the image. Such examination
should be performed by means of a magnifying glass or eye-piece B or a projection
lens reproducing on a larger scale the part o, o o₂ of the image to be examined. That
part of the image, however, is not, as the parts in proximity of the axis, situated at
right angles to the pencils of rays which produce it, but is inclined to this
perpendicular direction (indicated by a dotted line) at an angle u increasing with
the distance of the part o̟ o̟ o̟, from the axis of the lens system A, and as the axis of
the system B must approximately be situated in the direction of the pencils of rays
to be transmitted, this axis is likewise inclined at approximately the same angle u to
that direction, which is perpendicular to the image part o₁ 0 02. In consequence
thereof only a single line of the image part o̟ o̟ o̟, coincides with the focal plane of
the system B (the dotted line may represent also this plane), therefore only a narrow
strip of the said image part is distinctly reproduced by this system. With a photo-
graphic objective, the image surface F of which is approximately a plane, the
predescribed angle is equal to the angle between the axis of the objective A and the
principal ray of the mean pencil of rays producing the image part o, o 0.
In case
the field of reproduction of the objective system A extends through 60°, the angle "
amounts to 30° for an image part at the periphery of the said field.
To overcome the difficulty set forth, according to the present invention a prism P,
Fig. 2, is inserted into the path of the pencils of rays producing the image part
0002 This prism should be placed near the image surface F, preferably between
the said surface and the system A, and the refracting edge of it must be turned away
from the axis of the system A and be parallel to the image surface F, so that the axis
of the system A is situated in the principal section of the prisın. The pencils of rays,
the points of junction of which should be located in the image surface F at o, o o₂,
are deflected by the prism P and then joined in image-points o¹, o¹ o¹, situated on the
deflected principal rays of the pencils. As to the exact positions of the points o¹₁ o¹ o¹
it will be understood that the prism lengthens the pencils in proportion to the glass-
distance, i.e. the distance passed by the rays inside the glass body. Therefore in the
case of the mean pencil the distance N o' increased by a certain fraction of the glass-
distance M N, must be equal to the distance M. o. The said fraction of M N is
approximately .M N, where μ represents the refractive index of the prism, so that
μ
the total distance M N 0¹ exceeds the distance Mo by approximately
fl
.M N.
As the glass-distance M, N, is smaller and the glass-distance M, N, greater than the
glass-distance M N, the point o₁₁ is less distant from M, and the point o¹₂ more
distant from M, than the point of from M. The consequence is that the line o¹, o¹ o¹,
is less inclined to a plane perpendicular to the mean direction N o¹ of the deflected
pencils than the line o₁ o o, to a plane perpendicular to the mean direction M o of
the original pencils, that is to say: the prism P turns the image surface towards a
position perpendicular to the mean principal ray. Evidently it must be possible to
realize exactly this perpendicular position by applying a prism of suitable refracting
angle.
It is well-known that a deflecting prism combined with a lens system so as to
influence the central part of the image surface, causes deficiencies of the deflected
image on account of unequal displacement of the image points (see Straubel,
Ein Abbildungsfehler des Prismas, Wiedemann's Annalen, Bd. 66, 1898, pag. 346).
The present invention proves that, on the contrary, the prism may also serve for
removing deficiencies of reproduction when being adjoined to the lens system beside
the axis so as to receive only oblique pencils of rays.
A prism of a given refractive index μ exactly neutralizes, in the before-described
arrangement; an inclination u of the original image surface o₁ 0 0₂ to a plane.
perpendicular to the original direction of rays (Mo) by directing the new image
surface o¹ o¹ o¹ so as to stand at right angles to the new direction of rays (No¹),

page_text: if the refracting angle a of this prism corresponds to the following two pairs of
equations:
tan i
2+
These equations refer only to the angle of incidence i supposing that the position
of the prism is that of minimum deviation of the mean principal ray. The first
pair of equations relates to the image surface produced by the rays which travel in
the plane of principal section or primary plane, and the second pair relates to the
image surface produced by the rays which travel in the secondary plane, a plane
through the principal ray, perpendicular to the primary plane. The difference
between the values of a, and a, corresponds to the astigmatism the prism produces
on the different principal rays, according to their unequal glass-distances.
astigmatism is practically removed by giving the refracting angle a a mean value,
and Цз
between
at
This
The dispersion of colours produced by the prism is the less perceptible the nearer
the prism is brought to the original image surface o, o o; but it will be understood
that the prism may be achromatized in the well-known manner so as to fully do
away with the said dispersion.
The total deviation of the mean principal ray is derived from the refracting angle a
of the prism by a well-known formula.
The same effect that, in the arrangement just described, is produced by a
deflecting prism on account of the unequal glass-distances through which the
different pencils of rays travel, may also be had by a reflecting prism (Fig. 3) which
interposes a reflection between the two refractions and thereby imparts a deviation
of any degree to the mean principal ray. It will be understood, after a short
consideration, that in this case the perpendicular position of the new image
surface o¹, o¹ o¹, relatively to the final direction of the reflected mean principal ray
is obtained by making the angle y between the reflecting surface and the surface of
emergence larger than the angle ẞ between the reflecting surface and the surface of
entrance, and by making the difference between both angles equal to the refracting
angle a of the above-described deflecting prism. Moreover the smaller angle (8)
must be located next to the axis of the system A and the angle of incidence of the
mean principal ray must be equal to the angle of emergence and also equal to the
corresponding angles of the deflecting prism supposed to be in the position of
minimum deviation. The remaining particulars of the prison are determined in a
well-known manner by the angle w prescribed for the deviation of the mean principal
ray from its original direction.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
1. The combination with a lens system of a prism placed in the path of rays
beside the axis and near the image projected by the said system, and a second lens
system placed in the path of the rays which have passed through the prism,
essentially as and for the purpose set forth.
2. The combination with a lens system of a non-isosceles reflecting prism placed
in the path of rays beside the axis and near the image projected by the said system,
and a second lens system placed in the path of the rays which have passed through
the prism, essentially as and for the purpose set forth.
Dated this 6th day of April 1900.