Abstract: The present invention makes possible the quick, accurate analysis of images, especially medical diagnostic images of unknown magnification, to determine the size of objects visible therein, by reference to an object of known size or to determine the magnification ratio when it is unknown. There is provided a transparent image measuring device having one or more line pairs of varying spacing. Various scale markers are included on lines running substantially at right angles to the line pairs. The device is placed on the image over an object of known size (the reference object). The device is located so that the reference object just bridges between the spaced-apart lines in a particular location which is associated with a scale marker corresponding to the true size of the reference object.