sentences:  Until the second half of the 19th century, researchers had at their disposal three methods of investigating eye movement.The first, unaided observation, yielded only small amounts of data that would be considered unreliable by today's scientific standards.This lack of reliability arises from the fact that eye movement occurs frequently, rapidly, and over small angles, to the extent that it is impossible for an experimenter to perceive and record the data fully and accurately without technological assistance.