Improved Double Objective for Photographic and the like purposes.
Year: 1898

page_text: Date of Application, 8th June, 1898-Accepted, 6th Aug., 1898
COMPLETE SPECIFICATION
Improved Double Objective for Photographic and the like purposes.
We, CARL PAUL GOERZ and ЕMIL VON HOLGH, both of 45/46, Rheinstrasse,
Friedenau, near Berlin, Germany, Opticians, 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: —
Our invention relates to a double objective for photographic purposes
It is a well known fact that the anastigmatic Hattening and at the same time
the neutralisation of the spherical aberration in a system of three lenses 18 attained
by two spherical contact-surfaces of two succeeding mediums, one of them acts
as a collector, the other as a disperser for the luminous lays
J
Proceeding from the consideration that the efficiency of an objective of this
kind depends on the value of the difference of refractive indexes between the two
media in juxtaposition, we concluded that further improvements and especially
considerable increase in the sharpness of the picture would be obtained by
replacing the contact-surfaces between two different glasses by such between glass
and air
By this arrangement the difference between the refractive indices will
be increased six or seven times Researches in this line led us to the construction
of the objective which forms the object of the application, and which is formed
of two single lenses separated by a layer of air It is an objective of the very
simplest construction which in spite of its simplicity eliminates in a very high
degree all defects of the image produced, .e. spherical and chromatic abeiration,
astigmatism, convexity of the image, coma, distortion In addition to this, how-
ever, we have succeeded in considerably increasing the general sharpness of the
picture by diminishing the so-called "intermediate error as compared with the
astigmatically corrected cemented three lense systems hitherto used for the same
purpose
This two new lense system consists of a bi-concave lense L of low refractive
index and of a bi-convex lense L of a high refractive index as shewn in Fig 1
The present system 18 the final result of very careful scientific researches on the
following plan -
-
Equations were established for the elimination of the spherical aberration and
for producing a constant proportion of the sines for axial rays Different values
were introduced for the focal-length of the first negative lense, and for the
distance of the two lenses from one another The solution of the resulting
equation of the fourth degree had as a product the resp focal lengths of the second
lenses, and the forms for both lenses most apt for doing away with the two errors
Subsequently all these forms were controlled by tugonometrically calculating a
ray near the circumference taking into account the suitably selected thickness of
glass These calculations showed that the calculation of the correct thickness
of the glasses, which was originally left out of account only slightly influenced
the results necessary for the realisation of both conditions.

page_text: Each combination of lenses thus obtained was then examined for curvature of
the image and astigmatism, by projecting mathematically a principal ray inter-
secting the axis of the system under an angle of 32 degr The astigmatic points
of the image were then calculated The comparison of the relative aituation of
these points and of the plane of the image pointed to the way in which the
problem was to be solved
The systems studied in this manner all showed a considerable curvature of both
astigmatic curves of the image The conditions were the most favourable ones
in that system in which the focal length of the first lense was smallest Neverthe-
less, it would be necessary for a complete removal of this error to shorten the focal
length still further The result would have been a considerable decrease in
illuminating power in consequence of the great curvature of each lense
It was
therefore decided not to msist on the strict fulfilment of the ane-conditions in
the separate lense systems, and only bring the two astigmatic points of the image
in the imaginary plane of the image to a coincidence, the spherical aberration in
the axis being each time removed by trigonometric testing This was carried
out in the following way..
From the objectives calculated in the manner previously described we selected
one the curvature of whose lenses still admitted of a suitable aperture. By
varying the constant values, radius of curvature, thickness of the glass and distance
of the lenses, we tried to approach our aim This experiment succeeded com-
pletely although the departure from the strict fulfilment of the sine combination
was finally somewhat considerable.
Having found a kind of glass which approximately possessed the calculated
amount of refraction and of dispersing power for the spectrum and having made
the correction necessary on account of the small difference between the refractive
indices of the lenses obtained, and the values taken into calculation the testing
of the experimental objective in an apparatus specially constructed for that
purpose showed that in the single system the want of sufficient consideration of
the sine condition shewed itself in a not quite precise sharpness
On the other hand the advantages of the new system appear clearly when two
single systems are exactly centrally united to form a double objective as shewn in
Figure 3 This new double objective 18 formed to possess the same excellent
and astigmatic picture plane as the best double anastigmatic lenses hitherto known
and this moreover for a still larger opening This is explained by the compact
construction of the composite system and the thinness of the lenses employed
Moreover the considerably increased sharpness of the image of fine line objects
is remarkable both in the direction of the, axis and in that of main rays
interesting it at any angle
The constructional elements of a double objective of this kind are as follows. -
Radu R71 978 d - 2 246 Thickness of the lense L.
R$
Ra
-
-
88 944
180 924
59.309
R*
A
12 043
d²
-
1.813
Air space of both lenses
- 4 934. Thickness of the lense L
Air space between both single systems
The glass employed has the following optical properties
-
Lense L¹
-
1.5356
Refractive index for line D
Lense L
-
] 6112

page_text: By comparing the system of two lenses as described above to a system composed
of three lenses which form one single body, the advantages to the latter will be
readily recognised In Fig 1 and 2 there are two such systems illustrated side by
aude, that pattern of the well known astigmatically corrected three leuse systems has
been chosen, which mostly resembles the object of the present application with
regard to the duection of the radu of cuivature This system consists of a
positive Meniscus of low refractive index enclosed between two lenses of high
refractive index The figures inscribed on the separate lenses indicate in appioxi-
mate numbers the refractive indices of the glass employed As in all astigmatic
three-lense-systems, the astigmatic correction is obtained by a contact surface Z,
which disperses the light and another S concentrating it
The two lense system also contams two such contact-surfaces Z or S acting
in opposite sense but these are not contact surfaces between glass and glass, but
between glass and au It is obvious that, for a given aperture of a lense system,
the distinctness of the image after correction of the spherical aberation for the
peripheral ray increases with the increase of the difference between the refractive
indices at the contact surfaces The smaller these differences the deeper is the
curvature of said surfaces and the more the intermediate errors increase, that 18
to say, the spherical deviation of the lays which pass between the centie and
the periphery
These differences of refractive indices consequently in the three lense system
amount to:-
On the surface Z 006
On the surface S 010
In the case of two lense system on the contrary
On the surface Z 0'53
On the surface S. 061
consequently 9 01 6 times more respectively, and therefore the surfaces themselves
will possess a considerable flattening as compared with those of the three lense
anastigmatics. The very considerably increased clearness of the image is fully
explained by this cucumstance
===
The new two-lense-system can therefore be regarded as a system derived from
the three-lense-system by decreasing the power of refraction of the enclosed lense
until it becomes equal to the refraction of thean, te 1., and that it is no longer
a glass lense but an au lense enclosed between a bi-concave lense and a bi-convex
lense Obviously the bi-concave lense Figure 4, or the bi-convex lense (Figure 5)
or both (Fig 6) also can be composed of two or more lenses of equal or nearly
equal refraction, cemented together The condition sine qua non for getting,
for the above stated object, spherical, astigmatic and chromatic corrections with
increase in the clearness of the image, is formed by the following criteria
The back-surface of the first bi-concave lense of low refractive power has to
receive a deeper ourvature than the front surface of the bi-convex lense of high
refraction separated from it by a layer of air If for instance the achromatism
of the two systems of lenses can only be approximately attained with the available
species of glass we are able to cancel the still remaining error without any
difficulty by composing one of the lenses of two lenses of approximately equal
refraction but of different dispersion cemented together
Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim 18:-
An improved spherically, chromatically and astigmatically corrected two-lense-
system consisting essentially of a simple or composite bi-concave lense of low
refracting power and of a simple or composite bi-convex lense of high refraction
separated from the former by means of a layer of air, and in which of the surfaces
of the two lenses facing one another, that belonging to the bi-concave lense acts

page_text: as a disperser and has a greater curvature than the surface of the other lense
behind it, which acts as a collector, a single spherically, chromatically and astig-
matically corrected double objective being composed of two equal or similar halves
of the kind set forth
Dated the 9th dow of Tun 1004
W. P. THOMPSON & Co.,
322, High Holborn, London, W.C., Patent Agents for the Applicant