nice things, my new microscope has a built-in halogen
illuminator with a variable brightness control. I like
that, partly because at full brightness, my home-made
dark-field stop for the 40x objective works consistently,
proving that I made it correctly and that my old
illuminator, aimed at the mirror, just couldn't deliver
enough light. Hurray for built-in halogen. But then I
noticed, as I dimmed the illumination, that the color of
the light became increasing "warm" dim a
little bit, and it was yellowish; dim a lot, and it
became almost brownish. I removed the collecting lens
from the base of the scope and lookedthere was no
blue filter in back of the collecting lens, as there is
in my old illuminator. Thus I knew what would solve my
problem. But if I put a blue filter in the filter holder,
where would I put the dark-field stop? I tried balancing
the filter on top of the collecting lens: yes, it changed
the yellowish light to white, but it tended to fall off
whenever I touched it, and I found myself clumsily
touching it whenever I adjusted the mechanical stage or
the iris diaphragm. Surely, I'd grow less clumsy with
practice, but having a filter balanced so insecurely
seemed kind of primitive.
The solution to my problem was simple, and so satisfactory that I wanted to pass it along to others whose built-in illuminators produce light that's too yellowish: a styrofoam ring (cut from the bottom of a styrofoam cup) the outer diameter of which fits snugly inside the collecting-lens holder and the inner diameter of which is just smaller than that of a standard blue filter. Needless to say, this simple solution didn't occur to me right away, and I'll spare you the extra steps I took before I found that just the styrofoam ring (no other clutter) would do the job.
|Cutting the ring is the trickiest part. Use a narrow-blade scalpel for the inside cut, shave off less than you think you should on the outside of the ring, and take your time reducing the ring to the correct diameter. If you have to work fast, start off with several styrofoam cup-bottoms.|
|Next, find a spool of
thread or a prescription-drug container the diameter of
which is about the same as the filter's. Get some
"super glue" of the sort that will dry fast and
fill in tiny crevices. Then clean the underside of the
collecting lens and one side of your blue filter with
obsessive care (the other side of the filter can be
re-cleaned once everything is done).
Put the styrofoam ring on the spool, and then center the blue filter on the styrofoam ring, and then position the collecting-lens holder firmly over them. Now you have the filter centered against the collecting lens.
|Turn everything upside
down carefully to get the filter and styrofoam ring to
stay in place without help.
Make sure the filter and styrofoam ring are tight against the collecting lens before removing the spool.
|Now, tilt everything slightly (here's where the snugness of the ring is necessary) and apply a small drop of glue where the styrofoam ring and the collecting-lens holder meet. Repeat 180° from the first drop. Keeping the unit upside down, let the glue dry.||
Finally, after those two drops of glue are well dried and holding, apply more glue all around the circle so that the glue joins the styrofoam to the collecting-lens holder and almost covers the styrofoam. This last is to protect the styrofoam from the heat of the illuminator. Without the extra glue the styrofoam in time will curl downward and deform and eventually let go of the filter. With the extra glue, even if the styrofoam deforms, it won't deform downward it will shrink a little laterally but still hold your blue filter in position,
|... to give you whiter light.|
Comments to the author Rosemarie Arbur Comments to the author sent via our contacts page quoting page url plus : ('rarbur','')">email@example.com welcomed.
(If your web browser isn't set-up for e-mailing by clicking the above, just highlight and copy the address into your standard e-mail software).
Micscape Safety note: Styrofoam should be fine for microscope base lamps that don't get excessively hot at the condenser e.g. <90°C. If higher temps. are a possibility, a suitable heat resistant support is recommended (and a glass rather than gelatin/plastic blue filter should be used in either case).
Author's postscript: A more heat-resistant supporting ring could be made (less easily) of the plastic dish used for frozen microwavable entrees, the rolled edge of the aluminum foil tray used for frozen TV dinners, or a length of #12 or thicker insulated electric wire.
Published in the January 2001 edition of Micscape Magazine.
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