Image Micrometer.
Year: 1897

page_text: PROVISIONAL SPECIFICATION,
Double Image Micrometer.
"
We, ALBERT KÖNIG, of Carl-Zeiss-Strasse, at Jena, in the Grand-duchy of Saxe-
Weimar, German Empire, Doctor of Philosophy, and the Corporation "CARL ZEISS,"
a Company incorporated according to the laws of the Grand-duchy of Saxe-Weimar,
domiciled at the same place, do hereby declare the nature of this invention to be
as follows:-
In order to produce double images for the measurement of angles by means of a
telescope, every cone of rays entering the object glass has hitherto been divided
into two parts the line of division being either one of the diameters of the object
glass, so that two semi-circular halves were the result of division (as for example
in the case of the heliometer, Helmholtz's ophthalmometer and the sextant), or a
circle dividing the object glass into two concentric zones (as in Moser's double-
image micrometer).
Both these arrangements, however, are attended with the drawback that the
two images lying upon each other within the field of view of the telescope are pro-
duced by essentially different and totally separate portions of the object glass of
the telescope, and are made perceptible to the observer by different parts of the
cornea and the crystalline lens. As for this reason no identity of the dioptrical
conditions of production exists for both images, differences in their position and
size very frequently occur and interfere with the exact discernment of the coinci-
dence of both images. Besides, where these methods of dividing the image-produc-
ing pencils of rays are adopted, by any lateral displacement of the observer's eye,
in relation to the axis of the telescope, a portion of the rays of one of the images
will be shut off; by this circumstance the cause of errors above mentioned becomes
all the more apparent and the light is unequally distributed between the two images,
so that observation of coincidence is rendered very difficult indeed.
The invention hereinafter described obviates these inconveniences. According
to this invention each of the two areas of the object glass aperture which are
utilised in the production of one and the other image respectively is subdivided into
a great number of partial areas uniformly distributed over the whole aperture so
that every part of this aperture participates in producing both images. Such
distribution is rendered possible by means of an arrangement of mirrors used in a
well-known form of camera lucida.
Two plane mirrors are arranged in front of the object glass of a telescope, one of
which mirrors uniformly reflects the rays of light throughout the whole of its
area, while the surface of the other mirror is composed of a great number of parts
alternately reflecting and transparent, a result which may be readily obtained with
an ordinary foliated glass mirror, by removing many separate parts of the reflect-
ing coating. The transparent portions may vary in shape, for example they may
be streaks or circles, their arrangement also being optional (say in rows or irregular),
the only condition being that within the periphery corresponding to the effective
aperture of the object glass the total area of the transparent portions should be

page_text: approximately equal to the total area of the reflecting portions and that the centres
of gravity of these two groups should coincide as nearly as possible even though
the observer's eye were to occupy an eccentric position in front of the eye-piece.
As luminous rays from an object enter the object glass through the transparent
sections of the second mirior surface without any deflection, while, on the other a
hand, luminous rays from the same object are reflected first by the first mirror
surface and then by the non-transparent parts of the second surface, the object
glass, in its focal plane, will delineate two images, which will be more or less dis-
placed from their position of exact coincidence according to the angle of inclina-
tion existing between both mirrors. These images are produced at their coinciding
points by rays taking a practically identical path within the telescope and the
observer's eye, since to each ray contributing to the production of a certain point
of one image always belongs another ray which, starting from a closely adjoining
point of the object glass aperture, assists in the production of that point of the
other image which coincides with the said point of the first image. The two cones
of rays by which coinciding points of the two images are generated thus penetrate
each other; and they do so all the more completely, the smaller the alternately
transparent and non-transparent elements of the second surface, or the greater the
number of such elements.
The more thoroughly the two pencils of rays thus penetrate each other the more:
the ratio of the degrees of luminosity of both images will be independent of the
position of the observer's eye. As a matter of fact even with a comparatively small
number of alternately reflecting and transparent surface-sections, within that area
of the second mirror which corresponds to the object glass aperture, the partial
shutting off of the pencils of rays escaping from the eye piece by the margin of the
pupil, which is caused by an eccentric position of the eye, will affect the two images
practically equally.
The reduction in size of the alternating elements in the second specular surface
is somewhat limited by the aberration resulting from diffraction, which injures
equally both images, the diffraction being a consequence of the splitted sectional
area of the pencils of rays. Such aberration will not be perceivable in practice, if
the sections of the second mirror surface are of such a size, that their smallest
dimensions measured in that real image of this surface which, being reduced in
proportion to the magnifying power of the telescope, is projected into the pupil of
the observer's eye, are not less than 3 to 2 millimetres. However the second
surface may be divided into much minuter parts and the effect of diffraction, although
in such case being visible, may still be prevented from affecting the accuracy of
the observation by arranging the sections of the surface in streaks or bands with the
longitudinal direction of such streaks extending parallel to that diameter of the
field of view in the direction of which the images are to be displaced in relation to
each other for the purpose of angle-measurement. In this arrangement aberra-
tions owing to diffraction cannot have any injurious influence upon the observation
of coincidence, inasmuch as the spreading of light only takes place at right angles
to the direction of the separation of the images.
Again, the effect of a mirror surface split up into infinitely small elements may
be produced, and at the same time diffraction entirely eliminated, by arranging
instead of alternately reflecting and transparent elements elements of which each
is partly transparent and partly reflecting. This may be effected by foliating the
mirror glass with a very thin layer of gold or any other substance displaying powerful
superficial reflection, whilst still being translucent (when the layer is sufficiently
thin) for the brighter colours of the spectrum.
The arrangement substantially described above may be carried out by means of
ordinary plano-parallel glass mirrors each foliated on one side. In most cases
however it will be found advantageous to substitute prisms for both mirrors, or at
least for the second mirror. The first mirror is replaced by a single reflecting
prism of isosceles section, the hypotenuse side of which may either by free from
any coating, for total reflection, or coated with metal. For the second mirror an

page_text: isosceles prism should be substituted, the base surface of which is foliated as above
described, viz. either intermittently or with a uniform transparent coating. On
the foliated surface of the second prism a third similar but unfoliated prism is
cemented, so that the whole arrangement has in regard to the rays entering the
telescope without deflection, the effect of a plano-parallel plate.
When the combination of mirrors or prisms is intended merely for the production
of double images remaining in a constant relative position and is not to be used
as an instrument for direct assistance in the process of angle measurement, the
first and second. prism are preferably joined to form a single glass piece. The
double images which will by such means be obtained will be displaced in relation
to each other at a constant angle, which is independent of the position of the said
body of glass in relation to the telescope and only be determined by the angle of
inclination of the first or second reflecting surface in relation to the other. If in
this arrangement a measurable variation of the displacement of one image in
relation to the other is required, it must be brought about by means of a special
deflecting device (such as a rotatable prism or the like) inserted into the path of
the luminous rays in front of the compound prism.
It will be understood however that the combination of prisms or mirrors herein
described may also be utilised for continually varying the difference of direction of
both images and for measuring directly, on the principle of the sextant and the
prismatic reflecting circle, the said difference by constructing one of the two
mirrors or prisms so that it may be turned for measurable angles about an axis at
right angles to the optical axis of the telescope and parallel to the reflecting surface
of the other mirror or prism. For micrometric angle measurement in the special
sense of measurement of small angles the requisite rotation is preferably performed
with the second mirror or prism, whereas the first mirror or prism remains
stationary.
Dated this 28th day of Anal 1897.
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.
Double Image Micrometer.
We, ALBERT KÖNIG, of Carl-Zeiss-Strasse, at Jena, in the Grand-duchy of Saxe-
Weimar, German Empire, Doctor of Philosophy, and the Corporation "CARL ZEISS,"
a Company incorporated according to the laws of the Grand-duchy of Saxe-Weimar,
domiciled at the same place, 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:-
In order to produce double images for the measurement of angles by means of a
telescope, every cone. of rays entering the object glass has hitherto been divided
into two parts the line of division being either one of the diameters of the object
glass, so that two semi-circular halves were the result of division (as for example
in the case of the heliometer, Helmholtz's ophthalmometer and the sextant), or a
circle dividing the object glass into two concentric zones (as in Moser's double-image
micrometer).
Both these arrangements, however, are attended with the drawback that the
two images lying upon each other within the field of view of the telescope are pro-
duced by essentially different and, totally separate portions of the object glass of
the telescope, and are made perceptible to the observer by different parts of the

page_text: cornea and the crystalline lens. As for this reason no identity of the dioptrical
conditions of production exists for both images, differences in their position and
size very frequently occur and interfere with the exact discernment of the coinci-
dence of both images. Besides, where these methods of dividing the image-
producing pencils of rays are adopted, by any lateral displacement of the observer's
eye, in relation to the axis of the telescope, a portion of the rays of one of the images
will be shut off; by this circumstance the cause of errors above mentioned becomes
all the more apparent and the light is unequally distributed between the two
images, so that observation of coincidence is rendered very difficult indeed.
The invention hereinafter described obviates these inconveniences. According
to this invention each of the two areas of the object glass aperture which are
utilised in the production of one and the other image respectively is subdivided
into a great number of partial areas uniformly distributed over the whole aperture
so that every part of this aperture participates in producing both images. Such
distribution is rendered possible by means of an arrangement of mirrors used in
a well-known form of camera lucida.
Fig. 1 is a diagram showing the arrangement of the mirrors before the object
glass of the telescope.
Figs. 2, 3 and 4 are specimens of various delineations of the surface of one of
the mirrors.
Fig. 5 is an arrangement of prisms substituted to the mirrors.
Fig. 6 is another arrangement of prisms.
As represented in the diagram Fig. 1, two plane mirrors A and B are arranged
in front of the object glass of a telescope, one of which mirrors, B, uniformly reflects
the rays of light throughout the whole of its area, while the surface of the other
mirror, A, is composed of a great number of parts alternately reflecting and trans-
parent, a result which may be readily obtained with an ordinary foliated glass
mirror, by removing many separate parts of the reflecting coating. The trans-
parent portions may vary in shape, for example they may be streaks or circles,
their arrangement also being optional (say in rows or irregular), the only con-
dition being that within the periphery corresponding to the effective parture of the
object glass the total area of the transparent portions should be approximately equal
to the total area of the reflecting portions and that the centres of gravity of these
two groups should coincide as nearly as possible even though the observer's eye
were to occupy an eccentric position in front of the eye-piece. Figs. 2, 3 and 4
exemplify some methods of dividing the surface of the mirror A into a great
number of reflecting and of transparent sections.
As luminous rays from an object enter the object glass through the transparent
sections of the mirror surface A without any deflection, while, on the other hand,
luminous rays from the same object are reflected first by B and then by the non-
transparent parts of A, the object glass, in its focal plane, will delineate two images,
which will be more or less displaced from their position of exact coincidence accord-
ing to the angle of inclination existing between both mirrors. These images are
produced at their coinciding points by rays taking a practically identical path
within the telescope and the observer's eye, since to each ray contributing to the
production of a certain point of one image always belongs another ray which,
starting from a closely adjoining point of the object glass aperture, assists in the
production of that point of the other image which coincides with the said point of
the first image. The two cones of rays by which coinciding points of the two
images are generated thus penetrate each other; and they do so all the more com-
pletely, the smaller the alternately transparent and non-transparent elements of
the surface A or the greater the number of such elements.
The more thoroughly the two pencils of rays thus penetrate each other the more
the ratio of the degrees of luminosity of both images will be independent of the
position of the observer's eye As a matter of fact even with a comparatively small
number of alternately reflecting and transparent surface-sections, within that area
of the mirror A which corresponds to the object glass aperture, the partial shutting

page_text: off of the pencils of rays escaping from the eye piece by the margin of the pupil,
which is caused by an eccentric position of the eye, will affect the two images
practically equally.
The reduction in size of the alternating elements in the specalar surface A is
somewhat limited by the aberration resulting from diffraction, which injures equally
both images, the diffraction being a consequence of the splitted sectional area of
the pencils of rays. Such aberration will not be perceivable in practice, if the
sections of the surface A are of such a size, that their smallest dimensions measured
in that real image of this surface which, being reduced in proportion to the magnify-
ing power of the telescope, is projected into the pupil of the observer's eye, are
not less than 3 to 2 millimetres. However the surface A may be divided into
much minuter parts and the effect of diffraction, although in such case being
visible, may still be prevented from effecting the accuracy of the observation by
arranging the sections of the surface in streaks or bands, as illustrated in Figs. 2
and 3, with the longitudinal direction of such streaks extending parallel to that
diameter of the field of view in the direction of which the images are to be dis-
placed in relation to each other for the purpose of angle-measurement. In this
arrangement aberrations owing to diffraction cannot have any injurious influence
upon the observation of coincidence, inasmuch as the spreading of light only takes
place at right angles to the direction of the separation of the images.
Again, the effect of a mirror surface slipt up into infinitely small elements may
be produced, and at the same time diffraction entirely eliminated, by arranging
instead of alternately reflecting and transparent elements-elements of which each
is partly transparent and partly reflecting. This may be effected by foliating the
mirror glass with a very thin layer of gold or any other substance displaying
powerful superficial reflection, whilst still being translucent (when the layer is
sufficiently thin) for the brighter colours of the spectrum.
The arrangement substantially described above may be carried out by means
of ordinary plano-parallel glass mirrors each foliated on one side. In most cases
however it will be found advantageous to substitute prisms for both mirrors, as
illustrated in Fig. 5, or at least for the mirror A. Instead of the glass B, Fig. 5
shows a single reflecting prism of isosceles section, the hypotenuse side of which
may either be free from any coating, for total reflection, or coated with metal. For the
mirror A an isosceles prism a should be substituted, the base surface of which is
foliated as above described, viz. either intermittently or with a uniform transparent
coating. On the foliated surface of the prism a another similar but unfoliated
prism is cemented, so that the whole arrangement has, in regard to the rays
entering the telescope without deflection, the effect of a plano-parallel plate.
When the combination of mirrors or prisms is intended merely for the production
of double images remaining in a constant relative position and is not to be used as
an instrument for direct assistance in the process of angle measurement, the two
prisms a and b shown as separate in Fig. 5 are preferably joined to form a single
glass piece a b, as represented in Fig. 6. The double images which will by such
means be obtained will be displaced in relation to each other at a constant angle,
which is independent of the position of the body of glass a b in relation to the
telescope and only determined by the angle of inclination of one of the reflecting
surfaces A and B in relation to the other. If in this arrangement a measurable
variation of the displacement of one image in relation to the other is required, it
must be brought about by means of a special deflecting device (such as rotatable
prism or the like) inserted into the path of the luminous rays in front of the com-
pound prism.
It will be understood however that the combination of prisms or mirrors herein
described may also be utilised for continually varying the difference of direction
of both images & for measuring directly, on the principle of the sextant and the
prismatic reflecting circle, the said difference by constructing one of the two mirrors
or prisms so that it may be turned for measurable angles about an axis at right
angles to the eptical axis of the telescope and parallel to the reflecting surface of

page_text: the other mirror or prism. For micrometric angle measurement in the special
sense of measurement of small angles the requisite rotation is preferably performed
with the mirror A or prism a,. whereas the mirror B or prism 6 remains stationary.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed, we declare that what
we claim is:
1. For producing double images for the purpose of measuring angles the com-
bination with a telescope of a camera lucida consisting of two surfaces, of which
one is both reflecting and translucent and the other only reflecting, the first being
arranged before the object glass and the second aside of the first, the first surface
consisting of reflecting and transparent sections arranged alternately, or of a thin
layer of metal, substantially as described with reference to Figs. 1 to 4 of the
drawings.
2. In the combination described in Claim 1 the arrangement that the first surface
is situated between two prisms cemented together and shaped so as to present plano-
parallel outer surfaces to the rays of light, which pass through them in the direction
of the telescope, substantially as described with reference to Figs. 5 and 6 of the
drawings.
Dated this 1st day of January 1998
JOHNSONS & WILLCOX,
47, Lincoln's Inn Fields, London, W.C., Agents.