English: Single Lens Microscope, called "Wilson's screw-barrel microscope", after an English manufacturer who popularized it. Seven lenses (5 — 00) are shown at the bottom of the upper half. The smaller the lens, the more curved was the surface and thus the stronger was the magnification. For each lens, there was a special mount (b, q). Focussing was performed by turning the large screw against the spring. In the assembled state shown in the bottom half (with objects; strip with circles) the eye of the observer would be on the left side, an illumination on the right side.