Extracted from the 1924 Zeiss catalog:
Extracted from: John Belling, The use of the
microscope; a handbook for routine and research
work, 1930
The following description
of the principle behind this binocular eyepiece was
extracted from: Ophthalmic Literature,
Vol. XVIII, 76, 1922
Diaz-Caneja has
experimented with stereoscopic vision as observed
with the "Bitumi" of Zeiss. Helmholtz, describing the
stereoscopic microscope of Nachet, said that it allowed
of two images and that the instrument divided the
light, sending half to each eye. This, however, is
entirely different in principle from the instrument of
Zeiss, the "Bitumi." The stereoscopic problem of the
"Bitumi" is in reality the fusion of two unequal
images, or, in other words, fusion into simple vision,
of two points of disparity on the retina. To appreciate
the "relief" of an object it is necessary to perceive
the planes presented and to allow our visual line to
act as the stick which guides the blind man. If we have
to draw near or put away our fixation point, in using
the "bitumi," it is proof that the object has different
planes and offers a distinct relief. It is important in
using the "bitumi" that the eyes preserve their
parallel axes. In analyzing stereoscopic vision, it
must be stated that the visual space is a practical,
but not exact, representation of real space. We have
not only to see, but to interpret. Microscopic vision
with the "bitumi" is obtained with the use of both
eyes, without using the stereoscopic diaphragms, and
bearing in mind that the stereoscopic vision obtained
may not correspond in reality to the spacing of the
object.