on adapting microscopes for phase contrast
by Edward Cowen, UK
It is very easy today to purchase microscopes with built in systems for phase, but they are expensive for the amateur without access to these instruments in laboratories (e.g. where they work), and without finances available. Some good work can be done with the Russian and Chinese student or simple laboratory instruments. But they usually come with achromatic optics, which I find do not really allow higher magnifications essential in some cases. An alternative approach is to locate the better class semi-apo, fluorite phase objectives on their own from dealers such as Lindsey Optical (UK) or Herts Optical (UK) and attempt to adapt them to your microscope.
Both my excellent oil immersion phase 100X
objectives were purchased separately and I managed to adapt them
to my instruments. The first requirements were two centring
systems, one for the substage condenser, and one to centre the
sub-stage phase plate. Here the mechanically minded microscopist
would probably manage this, but my own attempts to do this
failed, as I could not get the accuracy required. The answer was
in the purchase of a second hand Zeiss phase condenser, again
from Lindsey Optical, which if it can be adapted to the substage
mounting is an ideal system to use. The condenser is a standard
size mounting and should fit into the substage.
Diagram of Zeiss phase condenser with top plate removed
The standard phase rings for Zeiss objectives with some luck may match the phase ring of the objective possessed, but the condenser phase rings can be scraped off if necessary.
Beutifully mounted, each disc is removable, slotting against a spring loading and individually centred with a key. A very versatile apparatus for adaptation.
Zeiss phase condenser with cover plate removed.
(The coloured discs of the homemade centre phase rings can be seen. The outer phase ring of each ring is formed by the condenser diaphragm).
The centre circular disc has 5 individually centred phase plates complete with inner circle and outer phase ring. If your objectives match these standard phase plates then you are lucky, if not this type of phase apparatus has one very good advantage over other types. The iris diaphragm forms the outer circle. On examination of the back lens of the objective with the eyepiece telescope one can see the inner phase circle and the iris in the same focus. This also allows some variation of phase. All one has to do is to alter the inner phase ring, with the use of a circle cutter, and a bit of tricky centring with the objective. (The inner ring can be cut out of thin card, and stuck over the existing ring, which makes the operation much simpler). Some skilful persons can cut phase rings with a slide ringing turntable and a fine brush, but this I warn is a very difficult thing to do, to match the two inner and outer rings, I would not attempt it.
A useful material for cutting discs (circle cutters can be bought from art shops) is to remove the film from old Polaroid films and use this; the film is coated with chemicals, which should not be touched with the fingers. This can be scraped off and then thoroughly washed, this makes ideal material for phase plates.
Comments to Edward Cowen welcomed.
A video clip taken by Edward of the beating cilia of a protozoan using phase contrast was shown in the January 1999 Micscape. Further video clips of Edward's work will be shown in later issues.
Published in the April 1999 edition of Micscape Magazine.
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