196
6. SECTION MOUNTING
Now a mystical ritual should be performed. The slide is held near
the open mouth and one vigorously breathes on it to "frost" its surface
with condensed water vapor. While still frosted, the slide should be
dipped into the water at an angle of about 30° from the horizontal. As
the dip starts, observe the end of the slide and be sure the film is loosening properly so that it floats off on the surface of the water as the slide
sinks down into it [Fig. 22(a)]. Have a toothpick handy which can be
wet, and the film pushed away from one's fingers after it has been floated.
A suitably thin film will be hardly visible, and certainly will not show
interference colors. It can be seen only by looking at it with reflected
light. A broad fluorescent light source is excellent for this. It is usually
convenient to have placed a specimen grid at each end of the film before
stripping it. These then serve as markers.
Specimen screens are placed in rows on the floating film [Fig. 22(b)].
Two precautions should be observed. If the screens have been punched in
their manufacture, they have a slight burr around their edges so that
one side is convex and the other concave. It is important that the convex
side be the side in contact with the Parlodion film. Otherwise the film
may stretch from the edges of the burr, and make only occasional contact
with the screen. The membranes then invariably rupture and the screens
are quite useless. In fact it is wise to bend the screens slightly to increase
the convexity in contact with the film. Also, rows of screens should not
be placed too close to one another for there must be reasonably large
expanses of exposed film to make contact with glass during the pickup
operation described next.
Before proceeding further, each floating screen should be tapped gently
with a needle so that the film is depressed. This assures a good bond
between the screen and film. Then one is ready to pick up the screens
from the water surface. This is done by taking a dry microscope slide
and, while holding it nearly horizontally, coming down on top of the
floating
film with its screens [Fig. 22(c)]. This motion should be fairly
fast to prevent condensation of water on the glass, which then would
not allow the glass and film to adhere to one another properly. The slide
is pushed right down through the water surface and when it is totally
submerged it can be turned over [Fig. 22(d)] so that the film with the
screens are on its uppermost surface. There is no danger of the film or
the screens floating away if the indicated precautions have been observed.
The slide with the film overlying the grids next is lifted out of the water
and drained. Large droplets of water in the vicinity of screens should
6. SECTION MOUNTING
Now a mystical ritual should be performed. The slide is held near
the open mouth and one vigorously breathes on it to "frost" its surface
with condensed water vapor. While still frosted, the slide should be
dipped into the water at an angle of about 30° from the horizontal. As
the dip starts, observe the end of the slide and be sure the film is loosening properly so that it floats off on the surface of the water as the slide
sinks down into it [Fig. 22(a)]. Have a toothpick handy which can be
wet, and the film pushed away from one's fingers after it has been floated.
A suitably thin film will be hardly visible, and certainly will not show
interference colors. It can be seen only by looking at it with reflected
light. A broad fluorescent light source is excellent for this. It is usually
convenient to have placed a specimen grid at each end of the film before
stripping it. These then serve as markers.
Specimen screens are placed in rows on the floating film [Fig. 22(b)].
Two precautions should be observed. If the screens have been punched in
their manufacture, they have a slight burr around their edges so that
one side is convex and the other concave. It is important that the convex
side be the side in contact with the Parlodion film. Otherwise the film
may stretch from the edges of the burr, and make only occasional contact
with the screen. The membranes then invariably rupture and the screens
are quite useless. In fact it is wise to bend the screens slightly to increase
the convexity in contact with the film. Also, rows of screens should not
be placed too close to one another for there must be reasonably large
expanses of exposed film to make contact with glass during the pickup
operation described next.
Before proceeding further, each floating screen should be tapped gently
with a needle so that the film is depressed. This assures a good bond
between the screen and film. Then one is ready to pick up the screens
from the water surface. This is done by taking a dry microscope slide
and, while holding it nearly horizontally, coming down on top of the
floating
film with its screens [Fig. 22(c)]. This motion should be fairly
fast to prevent condensation of water on the glass, which then would
not allow the glass and film to adhere to one another properly. The slide
is pushed right down through the water surface and when it is totally
submerged it can be turned over [Fig. 22(d)] so that the film with the
screens are on its uppermost surface. There is no danger of the film or
the screens floating away if the indicated precautions have been observed.
The slide with the film overlying the grids next is lifted out of the water
and drained. Large droplets of water in the vicinity of screens should