CHAP. XIV.] NICETIES OF TOUCH .WEIGHT. 431 



tinguished. That our power of varying the force of contact adds 

 much to the delicacy of touch, is evident from this : that a plane 

 surface may be made to seem concave, by drawing it over the 

 passive tip of the finger of a person whose eyes are covered, pro- 

 vided it be pressed at first strongly, then lightly, then strongly 

 again; or it may be made to seem convex by reversing these gra- 

 dations of pressure. But, if the individual himself is the regulator 

 of the pressure, the deception vanishes. We may obtain some 

 knowledge of the irregularities of surfaces, and the shape of objects, 

 by simply bringing the tactile organ into contact with them ; but 

 much more by moving it over them with attention. Thus, too, the 

 infinite diversities of texture may be made distinguishable by the 

 education of tact, combined with that of the muscular sense. It is 

 related of Saunderson, the blind professor of mathematics at Cam- 

 bridge, that he could distinguish a spurious from a genuine medal, 

 when the deception had imposed upon connoisseurs ; and the case 

 of the blind man, referred to by Rudolphi, who was able to dis- 

 tinguish between woollen cloths of different colours, of course by 

 some slight variety in their texture, is rendered credible by many 

 well-attested examples of a parallel kind. 



Our power of appreciating the weight of bodies, as well as resist- 

 ances in general, depends on those of estimating, separately and in 

 concert, both pressure on the tactile organ and the amount of con- 

 tractile energy acting in the muscles. Weber performed experi- 

 ments to ascertain how far we are capable of judging of weight by 

 the mere sense of contact. He found that when two equal weights, 

 every way similar, are placed on corresponding parts of the skin, we 

 may add to or subtract from one of them a certain quantity without 

 the person being able to appreciate the change ; and that when the 

 parts bearing the weights, as the hands, are inactively resting upon 

 a table, a much greater alteration may be made in the relative 

 amount of the weights without his perceiving it, than when the 

 same parts are allowed free motion. For example, 32 ounces may 

 thus be altered by from 8 to 12, when the hand is motionless and 

 supported ; but only from 1 \ to 4, when the muscles are in ac- 

 tion : and this difference is in spite of the greater surface affected 

 (by the counter pressure against the support) in the former than in 

 the latter case. Weber infers that the measure of weight by the 

 mere touch of the skin is more than doubled by the play of the 

 muscles. We believe this estimate to be rather under than over 

 the mark. 



The relative power of different parts to estimate weight corre- 



