a method of distinguishing between the front and back surfaces of a slide which includes an image on the front surface , comprises reflecting a light beam from each surface of the slide at an acute angle , measuring scattered light around the reflected light beam after the light beam has been reflected repeatedly , and comparing the measurements of the scattered light for the respective surfaces of the slide to detect which of the two measurements is larger . the surface whose measurement is larger is thus determined to be the front surface of the slide .