Answer: A simple microscope is a convex lens of short focal length that gives a magnified, erect virtual image of a nearby object.
When a tiny object is kept within the focal length of a convex lens, the lens forms a virtual, erect and magnified image of it. Watch repairers and jewellers have to see very small screws, gears and springs, so they use such a lens held close to the eye as a magnifying glass.