This instrument is signed
"R. Fuess, Berlin" with serial number 131. It is known
as the Fuess-Rosenbusch model and is an example of one
of the first types of microscope designed specifically
for petrological studies. The design was the idea of
the German petrographer H. F. Rosenbusch (1836-1914).
It dates from around 1878. The microscope is
described and illustrated in A Text-book of
Mineralogy by E. S. Dana, 1905 as follows:
"A highly serviceable
microscope, for general use, is that described by
Rosenbusch in 1876 and later much improved. A sectional
view of one form is shown in Fig. 516, and a later and
improved pattern is given in Fig. 517. The essential
arrangements of Fig. 516 are as follows: The tube
carrying the eyepiece and objective has a fine
adjustment-screw at g the course adjustment is
accomplished by the hand. The screw-head g is graduated
and turns about a fixed index attached to the tube p;
by this means the distance through which the tube is
raised or lowered can be measured to .001 mm. The
polarizing prism is placed below the stage at r, in a
support with a graduated circle, so that the position
of its vibration-plane can be fixed. The analyzing
prism is contained in a cap, ss, which is placed over
the eyepiece; this may be revolved at pleasure, its
edge being graduated. When both prisms are set at the
zero mark, their vibration-planes are crossed; when
either is turned 90ยบ, the planes are parallel (||). The
stage is made to rotate about the vertical axis, but
otherwise (in this simple form) is fixed ; its edge is
graduated, so that the angle through which it is turned
can be measured to 1/2 degree. Three adjustment-screws,
of which one is shown at n, n, make it possible to
bring the axis of the object-glass in coincidence with
axis of rotation of the stage (see, further, the
detailed drawing at the side).
The instrument here described may he used in the first
place as an ordinary microscope with magnifying power
adapted to the special case in hand. In the second
place, with polarizing prisms and the usual arrangement
of lenses, it serves for determining the planes of
light-vibration (like the stauorscope of Art. 328);
also for observing the interference-colors of doubly
refracting sections and so on. Finally, with eyepiece
removed and special condensing lenses added beneath the
object on the stage (as more fully described later), it
may be used, like the conoscope, for observing axial
interference-figures, etc.
The microscope which
has been briefly described is, as stated, especially
applicable to tlie study of the form, optical
properties, and mutual relations of minerals us they
are in thin sections of rocks; it has therefore become
an important adjunct to geological research. It can
also be used to great advantage in the study of small
independent crystals."