Microscope is an instrument which magnifies the size of an object so that things often invisible to the naked eye are rendered large and distinct by its aid. In its simplest form it consists of a single convex lens, either provided with a handle for ordinary use as a reading-glass, etc., or mounted on a stand, where it can be adjusted by suitable means till it is in the correct position for viewing an object. A simple microscope of very high power is obtained when the lens is part of a glass sphere around which a deep groove has been cut and filled up with black matter. In the compound microscope an inverted image of the object is formed by a lens or group of lenses, known as the object glass, and this image is viewed by the observer through another lens or group of lenses called the eye-piece. Below the object-glass is the stage upon which the object is placed. A hole in the centre of the stage allows light to be reflected from a mirror below, through the object if it is transparent, so that it may appear brighter. If the object is opaque, light is made to fall on it by means of lenses above. The instrument is generally focussed by first moving by hand the tube containing the object-glass and eye-piece, and then making a fine adjustment by means of a screw. The power of a microscope is altered by changing either the object-glass or the eye-piece, and larger instruments are provided with more than one of each.