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.
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