Natural Science Forum / Physics / Optics / December 2004

Hi Gregor,

You need to go back to my first response to Bob's question,. where  I
pointed out the most likely area to look for concentrations of
organisms.  He should include a little plant detritus (decaying
matter)  in the slide as the bacteria and the protozoa will tend to
remain in the area.

Now let me expand on why I see Bob's microscope as a problem in this
instance and why it is not as good as the system used by Leeuwenhoek .
I agree that the optics in Bob;s scope are slightly better than
Leeuwenhoek's   However,  Leeuwenhoek's microscope used a very
effective design to hang the specimen right in front of the magnifying
element of his microscope.  He did not have to chase after a target
that could swim quickly out of his field of view.  Bob on the other
hand, is working with slides which provide a vast area, relative to
his field of view,, in which the critters can swim..    Further Bob
does not  have the basic X-Y stage controls we take for granted on our
scopes.  The slide is held by simple clips and he needs to move the
slide with his fingers.  Even with X-Y controls it is tough to keep
the protozoans in the field of view at magnificationsnormally used  to
see reasonable details.  Without the stage controls, Bob is likely to
make vast random jumps across the field instead of being able to
systematically search the area under the coverglass. .

Better subjects for a beginner with this microscope would be mounted
insect parts, printed materials like money and stamps  These subjects
are stable and relatively large and would make use of the 4X and 10X
objectives..  

Its not that seeing protozoans is impossible with his microscope, it's
just a very difficult task.

Aaron .  .

Bob needs to find a sample with enormous concentrations of cells and
he would probably benefit from some gelling agent which slows the
critters down.  

To  Leeuwenhoek just seeing something swimming was the major
breakthrough. In order to keep Bob's interest and his kids interest I
am sure they want to see some internal details and the cillia
moving.etc.  A low magnification image is not going to do the trick.  

.  .    

There's also a seasonal aspect to this.  In the cold of winter many
protozoa encystize or go to the bottom.  Best bet is to do a visual
survey of the lake shore, and sample from areas where there is
evidence of vegetation or scum present, or where the water is
relatively still, or stagnant.  I find a large number of protozoa just
from the birdbath in front of my home, even in winter.

You cheap toy will do a lot more than a lot of people think. Concentrating
and staining the bugs is more important. You can improve the image some by
cutting down the stray light in the system with a simple stop below the
stage. Just a round hole that only lets in light that goes directly in the
objective.

It won't take good pictures but you can see a lot with them. You don't mind
taking them out in the field with you either.