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.