illustrated a further embodiment of a scanning probe adapted for use in the invention herein glass or lucite rollers 96 and 97 are each mounted for rotation about axes 98 and 99 respectively and loaded upward into the surface of palm 100 in a manner which would be obvious to those skilled in the art light sources 101 and 102 illuminate the palm by shining upward through rollers 96 97 the sides of rollers 96 97 are blackened at 103 104 to provide a dark surface for photocells 105 and 106 to focus against when the top inside surface of polished rollers 96 and 97 respectively act as internal reflectors surfaces 96 97 act as reflectors to photocells 105 106 when there is air in contact with them as shown when palm 100 is in contact with the surface of roller 96 photocell 105 34 sees 34 the light from source 101 which is reflected down into photocell 105 on the other hand when the surface of palm 100 is not in contact with roller 97 because of the presence of palm line light from source 102 does not reach photocell 106 because of the internal reflection caused by the air gap between palm 100 and roller 97 consequently photocell 106 sees only the darkened.