Draw a small circle on a white sheet of paper. About 6 cm (2 1/2 inches) to the right of this circle, draw a plus-sign, about 3 mm (1/4 inch) in size. Close your left eye and put your nose on the paper so that your right eye is directly above the circle. Slowly move your head up until you cannot see the plus-sign. Measure and record the distance from your eye to the paper. Move your head farther above the paper until the plus-sign reappears. Measure and record this distance. Why does the plus-sign disappear