250 
Proceedings of the Royal Society 
As a result of my experiments I am led to believe that, whereas 
the limitations in space are widely different, this does not hold with 
the limitations in time. 
If a rod vibrating about forty times in a second be held to the 
finger tips or lips, sensations of distinct impacts may be felt. 
Allow the rod to vibrate upon a surface less sensitive, and with 
niiicli wider limitations in space, say the skin over the sternum, 
and the impacts are still felt as such. I should mention that, 
inasmuch as the skin is less sensitive, the amplitude of the vibra- 
tions, and therefore the force of the impacts, should be increased. 
So far we have considered the limitation in time of tactile sensi- 
bility ; but there remains another very important function of the 
skin, which we have to discuss. I refer to thermal sensibility. 
On touching a good conductor, such as an iron ball, any difference 
of temperature will not at once be noted, the cold will not be felt 
instantly. On withdrawing the ball, the sensation of cold remains 
for a short time. The limits in time are very wide, for the applica- 
tion of the cold body on several successive occasions — even with 
intervals of a second — producing, of course, successive sensations of 
impact, gives rise to a continuous and uniform sensation of cold. 
If the good conductor be applied every second to the skin of the 
hack or arm, distinct sensations of cold will be produced each time. 
This is not due to these parts being covered and sensitive to cold, 
for the same obtains with the thin skin between the fingers, and the 
hack of the hand, although in a less degree. The difference 
must alone be due to the variations in thickness of the epidermis — 
a bad conductor — which separates the body touclied from the nerve 
end-organs in the lower layers of the skin. 
In studying these phenomena, I have used a very simple piece of 
apparatus. A long and slowly vibrating rod is fixed in an iron vice, 
and to its free end a small glass bottle is attached. This is fitted with 
a cork, through which passes an iron rod about half an inch wide. 
The bottle is filled with a mixture of ice and salt, which cools the 
rod in its whole length. The portion outside the bottle is caused to 
impinge upon the finger-tip periodically. 
In order that the action of external stimuli may affect our con- 
sciousness, a certain period of time is necessary. This period varies 
with many factors which we propose to discuss. A correct estirna- 
