of Edinhurgli, Session 1883-84. 
249 
roughness ” is not a sensation of intermittent stimulation. That 
this is its cause is hut a matter of experience aided hy the use 
of the other senses. A child blowing the sharp edge of a piece of 
tissue paper stretched between its lips, remarks that “ it tickles,” 
but is quite unaware of the cause of the sensation. Again, on 
touching the rapidly revolving wheel, it is conscious only of a con- 
tinuous sensation \ but turn it more slowly, and it feels each 
separate tap. 
We have then in tactile sensibility a condition analogous to the 
cause of dissonance in music, and to the annoying flickering of recur- 
rent visual stimuli. Moreover, we can recognise degrees or qualities 
of this “roughness,” a most important factor in adding to our know- 
ledge of the external universe. On drawing a file, or the edge of a 
fine saw across the finger, the sensation is quite difierent from that 
produced by the back of a knife. Moreover, within limits, we can 
distinguish a coarse from a fine file. The sensation in each case is 
different, and experience especially gained by the use of the eye, tells 
us of the nature of the substance which has produced the feeling. 
It is a well known fact that all parts of the skin are not equally 
sensitive. The skin of the finger-tips and the front of the hand 
can be stimulated by the impact of a lighter body than will affect 
the skin of the back of the hand. Again, the skin of the front of the 
arm is more sensitive than that of the dorsal surface, and still more 
so than the skin covering the back of the shoulders. Together with 
this difference in actual sensitiveness, the brain is unable to localise 
impacts which affect the less sensitive parts, so exactly as those 
which affect the more sensitive. As a result of this, tvro 
impacts made on points of the skin near enough may be fused into 
one in consciousness. If the points of a pair of compasses less 
than one millimetre apart touch the finger-tip they ma.y be dis- 
tinguished as two, the localisation of each point in consciousness 
being very exact. Over the back, however, the points must be 
removed for more than an inch before they can be distinguished. 
This difference, not to be discussed here, is probably due to the 
anatomical distribution of the peripheral nerves. 
It becomes an interesting question whether the limits in time of 
conscious tactile sensations vary in like manner over different sur- 
faces of the body. 
