intersection of the two straight lines designated by equation 4 and equation 5 indicates the coordinates of the affected part 8 and therefore the distance x to be calculated is shown by the following equation in this way the size u03b2 of the affected part 8 can be calculated by putting respective values into equation 3 further when equation 3 and equation 6 are previously formulated in a computer or the like the size u03b2 can easily be obtained only by inputting the respective numerical values further the indices made of metal according to the embodiment are not needed to stick to combinations of metal wires and metal balls but any indices may be used so far as they can be differentiated by observing an obtained image as other example it is conceivable to shift positions of indices on acrylic plates instead of providing the indices on the acrylic plates on the sides of light receiving portions respective numerical values of an obtained image photograph film or an image displayed on a monitor may be measured directly by a scale in that case distances between light receiving portions and measuring instruments are previously measured and a numerical value in place of the length l of the measuring instrument 1 may be used based thereon further although a method of fixing the measuring instrument according to the present invention at the head of an examined person is arbitrary a.