Abstract: An autofocussing mechanism is provided which is useful for all objective lenses and during scanning of the object. The autofocussing mechanism operates with a microscope having a main optical path, an objective lens, an object surface, an image plane and apparatus for changing the distance between the objective lens and the object surface thereby to focus the image of the object. The autofocussing mechanism preferably includes a pattern imaging system, a single image detector and a pattern focus analyzer. The pattern imaging system images at least one pattern through the objective lens along the main optical path and onto the object surface. The image of the pattern is then combined with an image of the object and is reflected along the main optical path towards the image plane. The image detector detects the reflected image and the pattern focus analyzer determines the extent of sharpness of the pattern by analyzing the output of the image detector.