Abstract: The illuminated object is brought to focus by a microscope objective lens in a focal plane. This image acts as the object for the lens under test. The position of the focal plane is adjusted by moving the object generator/viewing stage in the form of an autocollimator with a split beam eyepiece until it is coincident with the infinity focal plane of the lens under test. When this condition has been achieved, light transmitted by the test lens will be collimated. This collimated beam is reflected by a plane mirror and re-enters the test lens to be imaged in the focal plane and provide a well-focussed image as viewed by through the eyepiece. This image is also available for analysis using photoelectric techniques.