My favourite subject for microscopy is freshwater life and I prefer to see it live. The standard technique of using a slide and cover-glass doesn't give the specimens a lot of life expectancy. Use of a cell or ring slide is better in that regard but often has the snag that only low power objectives may be used as the front of the 40x would need to be below the cover glass!
A technique that I have found useful is to place a clean blank slide in the mechanical stage and then put a petri dish of interesting pond water on top. With the low power objectives, 4x and 10x, the lack of a cover glass will not cause significant aberrations. Higher power requires a waterproof lens.
A confession is in order here! I decided that what I needed was a "water immersion" objective and duly bought a Russian 40x with N.A. of 0.75 - a rather good lens. This I dunked in the petri dish, confident of seeing my specimens in a new light. Indeed, for the first few weeks this was fine but there came a sad day when I realised that everything was blurred. Inspection of the objective revealed that there was condensation inside - i.e. the lens had leaked and fungus had grown inside.
Thanks to the skilled help of a gentleman in the North my lens was rescued and is now used only as intended by its makers - i.e. as one would use an oil immersion lens, with just a drop of water between the front lens and the cover glass.
At this point I thought that my dunking days were over unless I could find one of the rare lenses built for deep immersion. However, I decided that it would be worth trying to waterproof a cheap second-hand 40x objective that I had. How? That was the question! The answer came in the form of a roll of cling film. I cut a small piece, about 7 - 10 cm square and placed it over the front of the 40x objective, being careful not to get bubbles or wrinkles where it touched the glass. I wrapped the remainder securely up and round the barrel of the objective and fitted it to the nose-piece of the microscope. (Note: If the objective is a sprung one don't compress the spring as you apply the cling film!)
In use, lower the objective into the petri dish of water until it makes contact with the water's surface. Further lowering of the objective (or raising of the stage) needs to be carried out with care.
As for the optical properties of the cling film I can say little. It is thin - about 10 microns thick - and doesn't seem to cause a colour cast. Anybody out there like to analyse the effect on the aberrations? My results have been quite pleasing and an example is shown above.
CAUTION:
I really recommend that you try this with an old objective rather
than risk your best plan apochromatics! Also, take care that the
water is free from grains of sand that could snag the objective
and damage it.
Happy dunking!
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