Other scientists had established that,
when a person’s eye was illuminated in a
dark room and the individual was adjacent to the light source, the eye seemed
to give off a red glow. (Today we observe
this as “red eye” in flash-photographic
images.) Helmholtz used geometrical
optics to analyze the refraction of the
light rays that are reflected back from
the illuminated eye. His analysis showed
that the light rays entering the pupil
and the reflected rays leaving the pupil
follow identical paths. A point source of
light (a candle or sunlight from a small
hole in the window shutter) forms a
point on the retina. The returning light
rays traverse the same path, but in the
opposite direction as the illumination
rays from the point source of light. All
the emerging rays return to the point
source of illumination. This explains why
an observer sees the interior of the eye as
black. Clearly, a special instrument was
needed to observe the retina.
In his initial experiments, he used
a candle as the source of illumination.
A glass plate was placed at an angle
between the subject and the observer.
The plate reflected the light from the
candle into the subject’s eye, and the
emitted light passed along the same path
as the illumination did, through the glass
plate and into the observer’s eye.
The problem with Helmholtz’s early
prototypes was that the resulting image
was blurred. This occurred because the
observer must be located very close to
the subject’s eye in order to see the retina
through the small pupil, and the emitted
light rays converge due to refraction in
the subject’s eye. Helmholtz’s solution
was to place a concave lens between the
observer and the glass plate—an adjustment that completed the invention of the
ophthalmoscope. Helmholtz called the
device the augenspiegel, or eye mirror.
Using it, he was able to see a sharp image
of a person’s retina.
Incidentally, Helmholtz was not the
first scientist to image the anterior eye—
but it was his design that turned out to
be the most useful and practical. For
example, in 1847, the British mathema-
tician Charles Babbage, known as the
inventor of the computer, devised an
instrument for imaging the back of the
eye; it contained a silvered mirror with
the silver removed in several spots for
illumination and observation. However,
Babbage could not correct the conver-
gent rays from the subject’s eye and
therefore he dropped the project. And
even earlier, in 1823, Jan Evangelista
Purkinje, a professor of physiology at
Breslau, had noticed that—under certain
conditions of illumination—one could
observe the back of the eye. However,
because he published the finding in
Latin, it remained unrecognized.
Müller inspires a
generation of biologists
The Anato- misches
Museum in
Berlin—where
Helmholtz
worked for a
year with anatomy professor
Johannes Müller by
Wikimedia Commons
Johannes Müller—trained
a generation
of future cell
biologists and
physiologists
in microscopic techniques and offered
its students a wide scope of intellectual freedom to pursue their research
interests. Müller’s students included
Jacob Henle, Theodor Schwann and
Robert Remak—all of whom contributed to cell theory—as well as Ernst
Haeckel, Rudolph Virchow and Emil
du Bois-Reymond.
Pasquale Baroni.
Helmholtz’s fellow students were
to become leaders in cell biology and
other areas within the biosciences,
perhaps due in part to Müller’s mentoring coupled with the intellectual
freedom they were given to pursue
their own curiosity-driven research.
Remak discovered unmyelinated
nerve fibers and the nerve cells in the
heart, and he revealed that cells originated from other cells, thanks to cell
division. Haeckel described many new
species and proposed a controversial
developmental theory. Virchow came
to be known as the father of cellular
pathology, and he was a great social
reformer as well. In the 1840s, both
Virchow and Henle were leaders in
Germany’s medical reform movement.
Du Bois-Reymond, a physician,
physiologist and long-time friend of
Helmholtz’s, discovered the action
potential in nerve fibers (i.e., the
propagation of the nerve impulse); his
research laid the groundwork for the
field of experimental neurophysiology.
Surrounded by so many fertile minds
and the excellent mentorship of Müller, Helmholtz’s own work thrived.
