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14
Point grid
From the diffraction image of the
point grid (Fig. 9), the 0th and the
first secondary maxima in the horizontal
axis are faded out via the slit.
In the intermediate image, a line grid
then appears in the perpendicular
direction (Fig. 11), and at an angle of
45° when the slit is oriented diagonally
(Fig. 12). A particularly remarkable
feature of the artificially produced
45° grating is that the lines are
closer to each other than in other images.
This is bound to be the case because
the secondary maxima in the
45° diffraction image are further
away from the 0th maximum than in
the perpendicular or horizontal diffraction
images.
With these experiments, Abbe finally
showed that the image in the
microscope is created in the space
between the primary diffraction image
(back focal plane of the objective)
and the intermediate image
plane through interference of diffracted
light waves.
In a further experiment, Abbe
demonstrated why the resolution
formula is
d=
�
2nx sin �
numerical aperture.
2 x n x sin � =
Abbe pushed the point-shaped light
source to the edge (known as
oblique illumination): therefore, the
1st secondary maximum can be twice
as distant as in straight illumination,
i. e. the distance d between two
points or lines can be twice as small.
On this basis, Abbe had already developed
the illumination apparatus
with focusing condenser in 1872 (Fig.
14). In today’s microscopy, we use unilateral
oblique illumination to achieve
full resolution with the maximum
condenser aperture.
The theory of microscope image
formation and its practical implementation
defined the resolution limits
and thus enabled the scientific construction
of microscopes. Abbe could
11 12
then concentrate on the correction of
spherical and chromatic aberration.
He realized that this requires the
development of special glass materials.
His collaboration with the glass
maker Otto Schott (1851-1935) started
in 1879, and the first Apochromat
objectives featuring high color fidelity
were already launched in 1886.
In 1893, August Köhler (1866-
1948) developed his illumination
technique with separate control of
luminous-field diaphragm and condenser
aperture on the basis of
Abbe’s results. Köhler also developed
the microscope with ultraviolet light,
which was primarily built to increase
the resolution by a factor of 2 relative
to green light. Finally, the phase
contrast technique from the Dutch
physicist Frits Zernike also is attributable
to manipulation in the back
focal plane of the objective.
With his theory and experiments,
Ernst Abbe decisively shaped the development
of microscopy in the 19th
and 20th centuries. Numerous Nobel
prizes are directly or indirectly con-
Innovation 15, Carl Zeiss AG, 2005