Other ways of manipulating
the light cone
For completeness, I have summarized all
the other techniques I know of for interfering with the light cone in order
to use it to transmit information to the lens and eye in various ways.
This is not a detailed account but maybe it will help yer understand that
you ain't just stuck with the methods I've described so far.
Phase Contrast
A bit like dark field (dark ground) but
expensive. An annular stop is normally built into the substage condenser
- much like our black disk. The expensive bit its that while this produces
a hollow cone of light - a special objective lens is used which contains
a phase ring. Rays of light, deviated by difference in refractive qualities
of processes in the specimen, miss the ring. The phase ring is made in
such a way that undeviated light passing through the phase plate (containing
the ring) is advanced by 1/4 wavelength. This results with deviated light
and undeviated light being 1/2 a wavelength out of phase and will cancel
each other out when they come together to form an image.
The result produces very contrasty specimens
against more brightly lit backgrounds.
Rheinberg's Differential
Color Illumination
A big title this one but the technique
is one you can exploit quite easily yerself. Yer remember our dark ground
method? Well this one is very similar. Here, yer simply replace yer black
disk for one which is deeply colored. Yer could use cellophane, or optically
sound transparent plastic or even stained glass. The outside space around
the central disk must be in another color. Commonly - dark red is used
for the stop and green for the surrounding area. Here you will be producing
a cone of light essentially two color: the central cone being red and the
external cone - green!
Oblique Illumination
A good playful one this. Here you simple
obstruct most of the light in the cone, leaving just the edge of light
cone still illuminating the specimen. You can put a piece of card in yer
filter holder ( a complete disc) and then slide the filter holder back
into place such that it leaves some light passing the obstruction.
You will get a false 3d shadow from your
specimen which gives the impression the subject is thicker than it normally
appears. Bear in mind that this is an illusion but it is an interesting
effect which can help reinforce that in the Microscopical world - things
are not really flat at all.
Closed Aperture
Illumination
If yer microscope
has an iris below the condenser, you can close it right down to form the
tiniest hole possible. This produces the effect of widening fine lines
and contours in the specimen (a contrast control?). Remember though,
that in doing this, yer not seeing how the thing really looks - merely
increasing the opportunity to spot the existence of detail where, previously
- none was visible.
Experiment
If yer (you are)
interested in taking photos or video of stuff under a microscope - it pays
to experiment with colored filters. Collect yer sweet wrappers if they
are colored cellophane, try different colors in the light cone path and
you will get more rewarding pictures. Remember to use color filters too
to correct the light for the type of film yer using and adjust the manual
white balance on video cameras.
That's it from me
for now. Our next project will be all about cutting sections manually for
mounting. Yer will need a microtome for this - either one yer already own,
or possibly made yerself. I'll be using the one I just bought from the
onview shop - The Brunel Bench Microtome.
See yer...
Larry.
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