THE TACTUAL ANALOG OF STROBOSCOPY. 
L E. DODD 
The subject of coincidences has application in stroboscopy, or 
the stroboscopic effect, in vision, and in the phenomenon of musical 
beats in audition. By means of a suitable mechanical device it ap- 
plies also to the tactual sense. There may thus be produced tac- 
tual, as well as musical and optical beats. 
A tactual sensation is aroused by simple pressure of a blunt 
point on the pad of a finger. This sensation may be given a uni- 
formly intermittent character through periodic change in the pres- 
sure. Such a periodic change is produced by, say, periodic com- 
plete removal of the point from all contact with the skin. If the 
frequency of contact is sufficiently high, the sensation itself may 
be expected not to have an intermittent character. This condition 
would correspond to the rapid flashing of a light before the eyes, 
where the frequency of the flashes is high enough, sixteen or more 
per second, so that the retinal after-image bridges over the time 
interval between flashes. It would involve a tactual after-image. 
Instead of the same blunt point pressing the finger-tip at each 
contact, there may be a succession of similar points having the 
successive contacts with the skin. The subject interprets the con- 
tacts as being due to the same point, just as in stroboscopy the eye 
interprets the successive similar flashes as due to the same source, 
whether this is true or not, as it usually is not. This is a funda- 
mental illusion in the stroboscopic effect. The illusion is present 
whether or not the frequency is high enough to impart a contin- 
uous rather than an intermittent character to the sensation. 
If, in the case under discussion, the successive blunt points have 
contact at slightly different places along a line proceeding in a defi- 
nite direction, the subject will interpret the sensation as due to 
the same point moving in that direction. 
A mechanical device producing this tactual analog must furnish 
the series of blunt points,^ — corresponding to stroboscopic “figures” 
(as distinct from the “images”), — and a means for bringing these 
into contact with the finger end, in the simplest case one at a time. 
The two characteristic frequencies will thus be found in that of 
the points, where points successively displace others, and in that 
