8.1.
INTRODUCTORY
REMARKS
267
2C microscope (which appeared in 1950) which first had this as part of
its regular equipment. The aperture was mounted between the lens
elements. Mechanically this presented very serious problems of optical
alignment, for the aperture, necessarily, had to be exactly in the path of
the main electron beam. Originally one had to remove the pole-piece
every time an adjustment was to be made. It might take a half day or
more of extremely vexing work to achieve the initial alignment of a new
aperture disk. In the meanwhile the chances were that the edge of the
aperture had become seriously contaminated, introducing other problems
which will be discussed later. This most annoying mechanical difficulty
was allayed only partly by the realization that a slightly loose aperture
disk could be bounced around, and so centered while the microscope was
operating, if one used a rubber mallet to wallop the column. Some
investigators become quite skilled at this ludicrous but useful maneuver.
Several independent efforts were made to devise a refined way of moving and aligning objective aperture disks while the microscope was in
operation. Most American microscopes at this time were RCA EMU-2
instruments, and in 1955 the Canal Industrial Corp. (Canalco), Bethesda,
Md., introduced commercially such a fitting for these microscopes. This
was, indeed, a most important practical development for us. The acceptance of this externally adjustable aperture holder was such that nearly
every electron microscope designed since that time incorporates a mechanism to allow external aperture alignment.
The use of very small objective apertures introduced a new difficulty,
for when their lips become asymmetrically contaminated, asymmetric
electrostatic forces appear. These, superimposed on the magnetic lens,
produce astigmatism which quickly deteriorates an image to the point of
uselessness. It had been recognized previously that the all-important
objective pole-piece could be "compensated" quite exactly by moving
FIG. 30. The same specimen of guinea pig marrow was micrographed before and
after a 50 u. aperture was put in the back focal plane of the objective lens, (b) Without
an aperture. The micrograph was printed from an adequately exposed but very flat
negative on the most contrasty grade of Kodabromide enlarging paper. The image is
very flat and weak, (a) With the aperture. Such a contrasty image was produced on the
negative that a softer enlarging paper had to be used. The central cell in these
figures
is a neutrophilic myelocyte (neut.). At the bottom of the figures from left to right are
the corner of a neutrophilic metamyelocyte, a normoblast (norm.), and a myeloblast
(m.'bl.). At the top of the figures is the margin of an erythrocyte (r.b.c). Specimen
fixed
in osmium tetroxide, embedded in methacrylate, unstained.
INTRODUCTORY
REMARKS
267
2C microscope (which appeared in 1950) which first had this as part of
its regular equipment. The aperture was mounted between the lens
elements. Mechanically this presented very serious problems of optical
alignment, for the aperture, necessarily, had to be exactly in the path of
the main electron beam. Originally one had to remove the pole-piece
every time an adjustment was to be made. It might take a half day or
more of extremely vexing work to achieve the initial alignment of a new
aperture disk. In the meanwhile the chances were that the edge of the
aperture had become seriously contaminated, introducing other problems
which will be discussed later. This most annoying mechanical difficulty
was allayed only partly by the realization that a slightly loose aperture
disk could be bounced around, and so centered while the microscope was
operating, if one used a rubber mallet to wallop the column. Some
investigators become quite skilled at this ludicrous but useful maneuver.
Several independent efforts were made to devise a refined way of moving and aligning objective aperture disks while the microscope was in
operation. Most American microscopes at this time were RCA EMU-2
instruments, and in 1955 the Canal Industrial Corp. (Canalco), Bethesda,
Md., introduced commercially such a fitting for these microscopes. This
was, indeed, a most important practical development for us. The acceptance of this externally adjustable aperture holder was such that nearly
every electron microscope designed since that time incorporates a mechanism to allow external aperture alignment.
The use of very small objective apertures introduced a new difficulty,
for when their lips become asymmetrically contaminated, asymmetric
electrostatic forces appear. These, superimposed on the magnetic lens,
produce astigmatism which quickly deteriorates an image to the point of
uselessness. It had been recognized previously that the all-important
objective pole-piece could be "compensated" quite exactly by moving
FIG. 30. The same specimen of guinea pig marrow was micrographed before and
after a 50 u. aperture was put in the back focal plane of the objective lens, (b) Without
an aperture. The micrograph was printed from an adequately exposed but very flat
negative on the most contrasty grade of Kodabromide enlarging paper. The image is
very flat and weak, (a) With the aperture. Such a contrasty image was produced on the
negative that a softer enlarging paper had to be used. The central cell in these
figures
is a neutrophilic myelocyte (neut.). At the bottom of the figures from left to right are
the corner of a neutrophilic metamyelocyte, a normoblast (norm.), and a myeloblast
(m.'bl.). At the top of the figures is the margin of an erythrocyte (r.b.c). Specimen
fixed
in osmium tetroxide, embedded in methacrylate, unstained.