I suddenly understood the shape and size of Leeuwenhoek's microscope. It was ergonomically designed to get as close to his eyeball as possible. I spent the next couple of days looking at different preparations, from pond water to fibers. Every slide that was observable using transmitted light gave great results. I was able to see the motions of rotifer cilia, the feet of fleas, and pigment particles in textile fibers.
Illumination is achieved by pointing the microscope at some source of incident or reflected light.
I used illumination sources ranging from bright clouds on a sunny day, to reflected sunlight from my house, to a frosted incandescent light bulb, to a cigarette lighter. Everything seemed to work great. Different wavelengths of light provide contrast suitable for different types of subjects. The only complaint was the aperture disk is somewhat difficult to rotate, however, this was a minor complaint since there is no real need to change its position frequently. Since the slide is inside the microscope, there has to be an easy means of moving the slide around during viewing.