The eyelids are first closed to protect the eyes from the sudden approach of a threatening body in the seventh or eighth week, although, as already observed, they will close against a strong light as early as the second day. The explanation of their beginning to close against the approach of a threatening body is supposed to be that an uncomfortable sensation is produced by the sudden and unexpected appearance, which causes the lids to close without the child having any idea of danger to its eyes; and the effect is not produced earlier in life because the eyes do not then see sufficiently well. On the twenty-fifth day the child first definitely noticed its father's face; when he nodded or spoke in a deep voice, the child blinked. This Professor Preyer calls a "surprise-reflex;" but definite astonishment (at the rapid opening and closing of a fan) was not observed till the seventh month. The gaze was first fixed on a stationary light on the sixth day, and the head was first moved after a moving light on the eleventh day; on the twenty-third day the eyeballs were first moved after a moving object without rotation of the head; and on the eighty-first day objects were first sought by the eyes. Up to this date the motion of the moving object must be slow if it is to be followed by the eyes, but on the one hundred and first day a pendulum swinging forty times a minute was followed. In the thirty-first week the child looked after fallen objects, and in the forty-seventh purposely threw objects down and looked after them. Knowledge of weight appeared to be attained in the forty-third week. Persons were first distinguished as friends or strangers in the sixth month, photographs of persons were first recognized in the one hundred and eighth week, and all glass bottles were classified as belonging to the same genus as the feeding-bottle in the eighth month. With regard to the sense of hearing, it is first remarked that all children for some time after birth are completely deaf, and it was not till the middle of the fourth day that Professor Preyer obtained any evidence of hearing in his child. This child first turned his head in the direction of a sound in the eleventh week, and this movement in the sixteenth week had become as rapid and certain as a reflex. At eight months, or a year before its first attempts at speaking, the infant distinguished between a tone and a noise, as shown by its pleasure on hearing the sounds of a piano; after the first year the child found satisfaction in itself striking the piano. In the twenty-first month it danced to music, and in the twenty fourth imitated song; but it is stated on the authority of other observers that some children have been able to sing pitch correctly, and even a melody, as early as nine months. One such child used at this age to sing in its sleep, and at nineteen months could beat time correctly with its hand while singing an air.