Hi again Rob,
I took another look at that stack.
http://www.kolumbus.fi/mimosa/HOLD/401-452.jpg
Much of what you interpreted as chromatic
aberration is the phase contrast halo. Phase
contrast is an imaging method that uses 1/4
wavelength 'out of phase' illumination to
enhance very small changes in refractive
index of a specimen. The colour is
unpredictable because not only does the phase
change, the wavelength does as well. There
are several filters in the lightpath and
changes in the voltage on the lamp also
complicates matters. There are too many
variables to control.
This specimen would be impossible to image
with phase contrast if it were not empty. The
protoplasm would simply make it too thick to
image and the whole thing would be a brightly
illuminated blob with nothing at all to be
seen. If phase contrast interests you there
is a very good explanation of it on the Nikon
microscopy website. The condenser provides a
mixture of light with a 1/4 wave 'out of
phase' component. This is imaged with an
objective that converts it back to 'in phase'
packets. But that's an oversimplification.
Its done with phase rings, or annuli if you
like, in the optics. The problem is that
thick specimens don't image properly. Thin
ones do. The bacteria in the picture -- dark
small and sausage shaped -- show up very
well. In normal bright field most bacteria
are practically invisible and are usually
stained.
Incidentally there are quite a few stacks to
be seen in the crystal pictures on my website
-- under the red 'Prints' button. I can't say
which ones off hand but those without 'out of
focus parts' are probably stacks.
D
--
Dr E D F Williams
__________________________________
http://www.kolumbus.fi/mimosa/index.htm
http://personal.inet.fi/cool/don.williams
See feature: The Cement Company from Hell
Updated: Added Print Gallery - 16 11 2005
