78 Dr Graves on the Sense of Touch. 



power the skin possesses (by means of touch and its modifications) 

 of comparing together two different temperatures or weights, 

 various and niuhiphed experiments prove that this power is ex- 

 ercised with the greatest success when the perceptions compared 

 are not simultaneous but successive. It is the same with the 

 smell, taste, and hearing; apply to the tongue by means of 

 camel's hair pencils, small portions of an acid and of a sweet 

 substance ; if the application of both be in quick succession, 

 their taste is accurately distinguished and appreciated ; but if they 

 be applied simultaneously, the result is a less vivid perception 

 of either, and a blending, as it were, together of the acid and 

 the sweet. A similar result is obtained by applying the mouth 

 of phials containing two different, but strongly odoriferous sub- 

 stances, to the nostrils ; and musicians have long ago remarked, 

 that when we wish to compare together two notes, it is done 

 with much more accuracy by striking them in quick succes- 

 sion, than by striking them simultaneously. Vision appears 

 to present an exception to the law which governs the other 

 senses ; for if we want to compare the lengths or the colours 

 of any two lines, we place them close together, and look at 

 them at the same moment. As Weber well remarks, how- 

 ever, the exception is here only apparent, for the truth is, that 

 we see nothing with perfect accuracy except its image Jails 

 on the retina at the extremity of the optic axis ; consequently, 

 on examining two lines close beside each other, although we 

 think we examine them simultaneously, yet we do not do so ; 

 our examination and comparison is made by causing the image 

 of each to occupy the extremity of the optic axis several times in 

 very rapid succession. The change in the position of the eye 

 is here so light, and is performed with such ease, that we are 

 unconscious of it. 



Weber made many experiments on the accuracy of the sense 

 of weight ; of course this sense is more developed in some indi- 

 viduals than in others, and is capable of being rendered more 

 exact by practice. Men accustomed to estimate weights by 

 poising them in their hands, will distinguish perfectly between 

 two only differing by a thirtieth part. In comparing two 

 weights, one is poised, and then instantly the other in the same 

 hand; the intervention of a few seconds between the poising 

 of the first and of the second, does not prevent their accurate 



