Microscopes (excluding electron microscopes) fall
primarily into two general categories - compound microscopes and
stereo microscopes (also called "dissecting microscopes"). Both
types range from fairly inexpensive to very expensive, depending on
features and quality (just like cameras or any optical equipment).
Compound scopes, cheap or expensive, are designed primarily to
provide high magnification, 2 dimensional views of very small,
transparent objects, like cells or bacteria. For shell work, a stereo
microscope is tye instrument of choice. These are designed for lower
magnification, 3-dimensional viewing of larger, opaque objects, say
from 1 mm or so upward - ideal for shells. The main differences
between the
expensive models and the cheap ones are lens quality (and therefore
viewing resolution and clarity), and various optional features, some
of which are valuable for shell work, and some of which are not. One
feature that is very useful for shell work (or any work for that
matter) is variable magnification. You can get stereo scopes with
one, two, or three different built-in magnifications, and also with
continuous zoom magnification from lowest to highest power. Each of
these steps translates into a substantial number of dollars.
Compound scopes and stereoscopes are really very different kinds of
instruments, and there isn't much you can do well with both.  Even
the best quality, most expensive stereoscopes just don't have the
magnification to study microorganisms.  That isn't their purpose.  On
the other hand, they do have the depth of field and working distance
necessary for examining shells, rocks, etc.; and also the proper
lighting for that purpose.  (They are also great for fixing jewelry,
removing splinters from fingers, etc!)
 
Magnification - The magnification of a microscope is equal to the
magnifying power of the eyepiece multiplied by the magnifying power
of the lower, or objective lens. Compound microscopes usually have
100x magnification at low power, and greater magnification at higher
powers.  40x final magnification is about the maximum magnification
you would normally need for shell work, and it's actually too
powerful for most shell work.  At 40x, a 2 mm shell is magnified to
almost 3.5 inches, which means you wouldn't be able to see the entire
shell in one microscopic field.  You can get a 20x magnification, or
possibly even a 10x on a compound microscope, but even these are not
too useful for shells, due to . . . .
 
Depth of field - Not being able to have the whole specimen in focus
at once may be tolerable for direct viewing - you can focus up and
down to see various parts of the specimen.  But, should you ever want
to connect a camera to your microscope, small depth of field would be
a real handicap.  For a good photograph, the whole subject has to be
in focus simultaneously.
 
Working distance - this refers to the distance between the objective
lens and the specimen.  In a compound scope, at 40x, that distance is
about a half inch, which makes manipulation of specimens rather
difficult.  You can't get your fingers in there, and if you use
forceps or tweezers there is a real danger of scratching the
objective lens.  Such a scope is designed for viewing slides, not
bulky objects.  At 400x, the working distance is only a couple of
millimeters.  In a stereoscope, the working distance may be as much
as 3 or 4 inches or more - plenty of room for hands and tools.
 
Lighting - A compound scope is designed to view
transparent/translucent materials by transmitted light.  The light
source is positioned under the specimen, and passes through the
specimen, forming the visual image.  A stereoscope is designed
primarily for viewing opaque objects by incident, or reflected light.
 The better quality stereoscopes have the light source in the head of
the instrument, with the direction of lighting essentially the same
as the direction of viewing.  When you place an opaque object on a
compound scope, the built-in light source of the scope is useless
because the specimen doesn't transmit light.  Therefore all you can
see is a dark silhouette of the specimen.  So, you have to provide an
exterior light source that will reflect off the upper surfaces of the
specimen.  This presents two major problems.  First, because of the
small working distance, the specimen cannot be efficiently
illuminated from above, so the illumination has to be from the side
of the specimen.  Secondly, because the objective lenses of a
compound scope are much smaller in diameter that those of a
stereoscope, they have much less light-gathering ability.  Therefore,
the specimen has to be very brightly lighted in order to see it well.
When the scope is used as intended, this is no problem, because you
are looking directly into the light source.  But trying to illuminate
an opaque specimen well enough to see it clearly by reflected light in
a compound scope is difficult.  You can't get a regular light bulb
close enough to the specimen, and a small penlight or flashlight just
isn't bright enough.  The only way to do it well is with a fiber-optic
light illuminator, which is an expensive piece of equipment itself -
and even then the above problems still exist.
 
Stereo effect - The other advantage of a stereoscope is the
three-dimensional effect it provides.  It has not only two eyepieces,
but also two matched objective lenses, so that each of your eyes views
the specimen from a slightly different angle, just as they do in
everyday life, creating a more realistic and three-dimensional image.
 In a compound microscope, even the expensive research models that
have two eyepieces, the image is essentially flat, or
two-dimensional.  In fact, flatness of field is a major objective in
designing such instruments.  This is fine for objects like cells or
microorganisms, which have negligible height or thickness.  But for
objects that are appreciably three-dimensional, it is desirable to be
able to see them as three-dimensional.
 
So, I think you'll really have to choose between shells (stereoscope)
or microorganisms (compound microscope), figuring that whichever type
of scope you get will enable you to do one of those two things very
well, and the other not very well, if at all.
 
Paul M.