Thursday, July 31, 2014


Out of the goodness of my heart rather than for any other cogent reason I occasionally glance at the microscopes of the local high school. For the most part microscope maintenance only comprises vacuuming bits of fluff off the eyepiece lens where they lie on the focal plane. Sometimes it includes cleaning the high power objective where the children plunge it into some aquatic biological specimen they are unable to see with the naked eye and are definitely unable to see when they use the microscope as Scuba equipment.

About ten years ago - actually it was nine according to my notebook - the school presented me with two identical wrecked instruments, and being
a) aware of the cost of school microscopes and
b) vicariously frugal
I cobbled the two together to make one intact and working one. Last week it was returned to me in a state where neither coarse nor fine focus would work. I pulled it apart and was interested to see what had happened to it in the interim.

Given that the lab technician doesn't dare touch the innards of the instruments and the schoolchildren don't have time I believe I may be able to surmise who might have effected - ahem - certain alterations to it. I offer the following hints to biology teachers around the globe as a consequence.

1. If you really do not know what you're doing, it can sometimes be wise not to do it.
2. If in spite of  1 (above)  you do inadvertently pull a microscope apart, it's quite a good plan to make detailed notes of every stage of disassembly so you know how to put it back together again afterwards.
3. Cheap school microscopes are still expensive.
4. Some instruments have the coarse focus pinion mounted inside an eccentric which is arranged to twist. If it twists one way, the rack becomes tight and immoveable. If it twists the other way, the stage falls out of focus and nothing but improvised props of petri dishes and other laboratory detritus will hold it in focus.
5. If, in an endeavour to correct the problem without recourse to knowledge or understanding you tighten up some suitable-looking screws VERY HARD, it is possible that the die-cast dovetail block will crumble.
6. If on reassembly you find several random parts left over, it's not necessarily best to put them in an icecream carton in a drawer somewhere else, and hope that tightening the screws VERY HARD will obviate their use.
7. And if you have removed any apparently surplus parts, it's as well to hand them over when asking someone to look at the instrument for you. Often the designer had some purpose in mind when he included a fine adjustment focus return spring and cap. Often he didn't simply pop them in there on a whim, or as a fashion statement.

I do hope these hints prove useful. There are a good many websites (warning: pdf) devoted to cleaning optical instruments, but there is one hint that I would add. When a lens is dirty, the dirt itself comprises some things (grit, fluff) and some stuff (oil, moisture) acting as a mordant. If you rub a tissue on the lens, the things (grit) are not miraculously removed from the stuff (oil), but rather are miraculously turned into a grinding paste. The lens designer would not thank you any more than Smetana would thank me for trying to play the Moldau on the mouth organ.

Corrosion. Nice.
Oh, and it's quite a good plan to keep your microscopes in a dry cupboard rather than the groundsman's potting shed.



Post a Comment

Subscribe to Post Comments [Atom]

<< Home