of Edinhurgh, Session 1883 - 84 . 
247 
affect the finger ti|) in a second, before the sensations are com- 
pletely fused in consciousness ; and before it becomes indistinguish- 
able from a revolving smooth metal disc. 
It is, then, to be noted that stimuli, separate in themselves, are, 
when sufficiently rapid, formulated in consciousness as continuous 
and uninterrupted ; and that this fusion occurs sooner in the 
consciousness of some sensations than in others. 
Some experiments which I have lately performed shed, I think, 
some light on these facts, and enable us to state them in greater 
amplitude ; above all indicating where the fusion occurs. I n)ay 
anticipate by stating that this is during the transformation of the 
stimuli into nerve energy. 
My experiments have been chiefly concerned with the investiga- 
tion of the results in consciousness of repeated stimulation of the 
nerves of tactile and thermal sensibilities, the apparatus being usually 
of the simplest possible kind. 
The skin was stimulated interruptedly in many ways, either Ijy 
tuning forks of various degrees of pitch, or by a revolving toothed 
wheel. The most satisfactory results, however, were obtained by a 
vibrating steel rod fixed in an iron vice. By altering its fixed 
point, and therefore the length of the vibrating portion, the period 
of its vibrations, and the impacts made upon the finger tip held near 
its free end, could be altered at will. Its exact pitch was easily 
obtained by attaching to it a light writing style which recorded its 
vibrations on a revolving smoked cylinder. 
If the finger be pressed lightly upon a revolving toothed wheel, 
whose motion is made to increase slowly in rapidity, the following 
sensations may be noted. When no more than forty or fifty taps 
are made upon the finger in a second, there is produced a conscious- 
ness of distinct stimuli, separated one from another by periods of 
rest. Each stimulus produces a distinct sensation, limited in time 
from the one that follows. Stimulate more rapidly, and it becomes 
no longer possible to distinguish in consciousness one stimulus 
from another. They give rise, however, to a sensation which we 
term “ roughness,” which becomes less marked — less rough — as we 
increase the rapidity of the revolving wheel. If, at this point, the 
fingers be very gently pressed against the wheel, there is a sensation 
of tickling — such as tliat produced by drawing a feather over the 
