There are digital USB microscopes available for very low cost from merchants on eBay. I bought one with high hopes, but I was disappointed when I tried to use it. Due to the high degree of magnification, the field of view is extremely small, and the depth of focus almost non-existent. This meant that the scope needs to be held rigidly for it to be useful. Using the stand that came with the microscope, it was almost impossible to adjust the focus without moving the scope so that the specimen wasn't even in the field of view any more. And if you tried to move the scope to aim it, you would inevitably change the focus. In addition, the stand included with the scope held it at an angle with respect to the table top. Given the very shallow depth of focus, the microscope needs to be held at right angles to the plane on which the specimen rests.
What I needed was a stand which would do three things:
1. Hold the microscope so that it is perpendicular to the table.
2. Hold the microscope rigidly.
3. Provide a focusing knob, so that the focus can be adjusted accurately without disturbing the specimen.
I wanted a small rack-and-pinion mechanism for the focusing. I considered several options, including re-purposing parts from an old photo enlarger. In the end, it was the same vendors on eBay from whom I bought the microscope that provided the missing parts. I got a macro focusing rail that was very inexpensive and fit the bill perfectly.
With the microscope attached to the focusing rail, all I needed was a rigid stand to hold it in position. I built a stand out of laser-cut acrylic. (I made it at TechShop. www.techshop.ws
) The result is a handy microscope that's easy to use and cost well under fifty bucks.