I. Jahresfeier am 30. Mai 2015

fessors of Heidelberg University and picked out two whose interests seemed most
closely related to the subject.
I wasn’t familiär with either of them. One was Reinhard Neumann, a lecturer
from Prof zu Putlitz’s chair on atom spectroscopy; he asked me whether I wanted
to do near-field optics. I replied with ‘far-field only’, whereupon he looked at me
with a Stare. But he read my essay and wrote a letter of support. The other was
Professor Christoph Cremer, who worked on flow cytometry and chromosome
Organisation, the only biophysicist in the directory. He also read my little essay with
interest. When I came back a few days later, he was excited and showed me a paper
of his from 1978, which he jokingly referrcd to as aJugendsünde, i. e. a peccadillo of
youth. The paper suggested a hypothetical hologram producing an elliptical wave-
front which was predicted to converge in a single point of light that would possibly
become infmitely sharp, thus breaking the diffraction barrier in a point scanning
configuration. He called it the “4rt microscope”. I instantly realized that even ifyou
could realize the hologram, the laws of physics would yield something eise. But
Professor Cremer was very supportive too, and wrote the other letter. The stipend
was later approved on condition that I spend six months abroad. I opted for Ox-
ford, to work with Professor Tony Wilson, an early confocal microscopy pioneer.
I fmally did that in 1995.
The EMBL was a great place. It was international, and the working language
was English. I took advantage of this time to learn English, and after I had listened
to many presentations, I eventually plucked up the courage to present in English
myself. I had no choice after all. With Ernst Stelzer I had agreed to build the mi-
croscope with the two counter aligned lenses to see if the axial resolution increase
could be realized. It wasn’t easy, and I remember that in 1991, one day before
my birthday, I had the first clear indication that it was feasible. The key was that
I could exactly predict what the experimental data should look like, so I was able to
discriminate against misalignment. In the publication, Ernst suggested that we call
it the “47t microscope”, which I wasn’t particularly happy about. For one thing, the
solid angle of the double lens arrangement was far from 4n. And then, the actual
discovery was that ‘4uj’ wasn’t needed to increase the axial resolution; two high
angle lenses were sufficient. Moreover, the paper that had coined this name had
not drawn the right physical conclusion and had missed out the axial resolution
increase (as the actual benefit) completely. Ernst and I finally compromised not to
use the Greek letter 7t, but the Roman letters Pi. Whether I liked it or not, the name
4Pi was stuck. The group was later reinforced by two talented physics diploma stu-
dents, Gernot Reiner and Steffen Lindek. Since Ernst did not have the habilitation,
thesis works were officially handled by Professor Cremer who became increasingly
supportive and interested in the topic.
In this quest for increasing axial resolution using two lenses, it was not enough
to produce a focal interference pattern with counter propagating waves. The chal-

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