12.3 Setting up Phase-Contrast MicroscopyBack
14th and 21st Feb See timetable
Phase contrast provides the quality image enhancement which is essential for the parasitology practicals.
It visualises internal detail of living cells and even enhances the image of fixed and stained preparations.
Phase contrast microscopes are extremely expensive and it is critical that you learn how to set up their optics to see what they can provide. There are two principal types of microscope Nikons, Swifts.
(Nikons and Swifts)
Note that on Nikons not all objective lenses are phase lenses. Check for coding on the objectives (marked with engraved rings near the lens end and the letters PH) before you proceed. 1. Place a slide on the stage.
2. Focus on the specimen at X 10.
3. Close the sub-stage diaphragm (at the light source)
4. Focus on the vanes of this diaphragm with the condenser adjustments knobs. (Ask if you don't know which they are).
5. Centre the focussed diaphragm and at each magnification. Spread the vanes until the diaphragm disappears (symetrically) just beyond the field of view.
To achieve this continue centreing adjustments as you open the diaphragm. Adjustments are made with the appropriate controls (right and left adjustments on Nikons) or by pushing the light source lens (Swifts).
Now for the phase rings: One in the objective lens and the other in the condenser apparatus. These must be made concentric with as much accuracy as you can achieve.
6. Insert the phase telescope in place of the/an eyepiece and twist out the telescope at its top focus on the ring in the objective lens.
7. You will find as a part of the condenser a wide thin cylinder which you can rotate to bring into position the appropriate (sub-stage) phase plug with its ring (Colour coded on the Swifts to match the colour on the objective lenses or marked with numbers on the Nikons).
8. Looking through the telescope make the lower ring perfectly concentric with that in the objective lens by pushing the phase plug using the first finger and thumb of both hands.
9. Swap the telescope for the eyepiece and you will be able to view your specimen freely.
N.B.The initial stages of setting up phase (Nos. 1-5 below) can only be carried out at X 10 mag. on Swift microscopes. It is therefore essential that this has been done at X 10 before proceeding with the other steps at the higher magnifications.
NB. This entire process must be repeated at each magnification with Nikons because the phase plugs become misaligned when microscopes are handled between practical classes. They are essential at the beginning of every practical to exploit the full potential of these microscopes.
© Dr. Clive Bennett 2006