Abstract: A method and apparatus for measuring the wall thickness of transparent objects, such as glass tubes, is presented wherein a narrow band (beam) of light is projected onto a surface of the object at an angle other than 90.degree.; and the beams reflected by the adjacent boundary surface of the object and that boundary surface thereof which is spaced behind that adjacent boundary surface by the distance to be measured are focused onto the image plane of a receiver. A movable deflecting element is provided in the optical path between the light source and the image plane of the receiver. The lines of intersection between the band of light and the boundary surfaces of the layer to be measured (the lines of intersection being represented by bands of light on the image plane of the receiver), are then moved or urged by the deflecting element in the direction in which they are spaced apart.