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Proceedings of the Royal Society 
face. If, as a final step in tlie experiment, we cause the wheel to 
revolve very rapidly, the sensation becomes less rough, and, at last, 
when as many as 1400 impacts are made upon the finger tip, the 
sensation cannot be distinguished from that of a revolving toothless 
metal disc. There is, in fact, a sensation of ‘‘smoothness,” which, but 
for the shearing produced upon the skin, could not be distinguished 
from the “ touch ” of a stationary smooth metal surface. We see 
then that three distinguishable conditions are produced by stimu- 
lating the skin more and more rapidly. At first, each stimulation 
is formulated separately in consciousness, then we have a sensation 
of roughness, and finally one of smoothness. 
Before proceeding further it will be well to insist upon the fact, 
that when we consider the subject more generally, we can find 
analogous conditions in the other sensations. There is certainly in 
hearing and sight, for instance, a period of “ roughness,” which is 
irritating in its nature, and is a distinct sensation preceding the com- 
plete fusion of the stimuli in consciousness. Moreover, the nature 
of its cause is known only by experience gained from other sensa- 
tions ; it not being evident in the consciousness of the special 
sensations above mentioned. 
The tick of a watch is not irritating — on the contrary, it may 
produce hypnotism. An alarum with more rapidly repeated 
sounds has a well known effect, upon which it will be needless to 
insist. My colleague, Professor Poynting, suggests that the very 
low notes of a large organ are, in like manner, irritating from their 
rapid intermittence. In the numbers quoted from Helmholtz, pro- 
bably the “ beats ” are no longer separately heard, but produce the 
harsh dissonance which characterises them ; a much lower number 
would indicate the number of stimuli which can be heard separately. 
This number I am unable to state, as a double syren is not in my 
possession. 
Pew things are more annoying than the flickering of a gas jet, or 
the slow vibration of a body which gives a blurred unsteady image. 
The tickling of a vibrating tuning fork held to the lips, or still 
more the tickling that results from stimulating in rapid succession 
adjacent points of the skin, is calculated in extreme cases to give 
excruciating agony. 
A few minutes’ consideration will also show that this condition of 
