Dr Graves on the Sense of Touch, 79 



comparison. The interval may amount to twenty seconds, and 

 yet a just estimate will still be made; but when it amounts to 

 forty seconds, all accuracy is lost. The sight enjoys a still more 

 accurate power of discrimination than the sense of weight, for a 

 well practised eye will distinguish between two lines one hun- 

 dred, and one hundred and ope lines long respectively ; in other 

 words, will discover a difference amounting to one-hundredth 

 part of the whole. According to the experiments of Delezen- 

 nius, quoted by Weber, the sense of hearing is still more accu- 

 rate, for a well practised musical ear will distinguish between 

 two sounds differing from each other only jj^, calculating the 

 number of vibrations the sounding bodies make in a given 

 time. 



A line can be perceived to be longer than another, even when 

 an interval of fifty or sixty seconds elapses between looking at 

 the first and at the second, provided that the lines differ j^th 

 in length. If they differ only ^th, then an interval of thirty- 

 five seconds may elapse without destroying our judgment ; but 

 if it be longer, our judgment becomes incorrect. When the 

 difference between the lines amounts only to g^th, an interval of 

 three seconds between the examination of each is the longest 

 that can be allowed without interfering with the correctness of 

 the comparison. Having followed Weber with some accuracy 

 through the body of his valuable treatises on the Touch, it may 

 be worth while to dwell again, for a moment, on some of the 

 chief conclusions he arrives at. We have a well established and 

 definite idea of the distance of some parts of our bodies from 

 others. Thus we feel the distance of the finger points from the 

 wrist, and we remember that distance. It is the same with the 

 arms as far as the elbow, and with the foot. These are all 

 lengths which are firmly imprinted on the mind, and conse- 

 quently there is a physiological reason for using them, as man- 

 kind have always done, as standards of measurement. When 

 any two points on the surface of these parts are touched at the 

 same time, we can with our eyes shut, and by means of the sense 

 of touch alone, guess with great accuracy the distance the touched 

 points are from each other, provided the points are situated 

 somewhere near the sides or extremities of these parts, as at the 

 tips or on the sides of the fingers. Here two points will be per- 



