In a simple optical microscope, the components includes the eye piece or ocular, objective lens, stage and illumination source. All optical microscope share the same components. A cylinder containing two or more lenses to bring the image to focus for the eye is the eye piece or ocular. It is inserted to the top end of the body tube, it is interchangeable and many other eyepieces can be inserted with different magnifications. The typical values for eye pieces include X5, X10 and X20 that are matched to give the best possible optical performance. The objective lens on the other hand is a cylinder containing one or more lenses to collect light from the sample. These lenses are screwed into a circular nose piece at the lower end o the microscope that can be rotated to select the required objective lens.

Typical values objectives are x4, x5, x10, x20, x40, x80 and x100. The stage is a platform below which supports the specimen being viewed. The illumination source, below the stage the light is provided and in a controlled variety of ways. At simplest, light is directed via a mirror. During the time of Anton van Leeuwenhoek, his microscope consisted simply of a screw used for adjusting height of the object being examined, a metal plate serving as body, a skewer used to impale the object and rotate it and lastly the lens itself which is spherical. Commonly the simple microscopes are double convex or called Plano convex lenses, or system of lenses acting together to form the image. A magnifying glass, an ordinary double convex lens, having a short focal length, is a simple microscope. The reading lens and the hand lens are instruments of this type. When an object is placed nearer such a lens than its principal focus, an image is produced that is erect and larger than the original object.