4 mm. long. But when the distance is 15 mm. the impression is less
like that of a solid body but still not ordinarily like two objects.

In connection with the subject of diffusion the _Vexirfehler_ is of
interest. An attempt was made to develop the _Vexirfehler_ with the
æsthesiometer. Various methods were tried, but the following was most
successful. I would tell the subject that I was going to use the
æsthesiometer and ask him to close his eyes and answer simply 'one' or
'two.' He would naturally expect that he would be given part of the
time one, and part of the time two. I carefully avoided any suggestion
other than that which could be given by the æsthesiometer itself. I
would begin on the back of the hand near the wrist with the points as
near the threshold as they could be and still be felt as two. At each
successive putting down of the instrument I would bring the points a
little nearer together and a little lower down on the hand. By the
time a dozen or more stimulations had been given I would be working
down near the knuckles, and the points would be right together. From
that on I would use only one point. It might be necessary to repeat
this a few times before the illusion would persist. A great deal seems
to depend on the skill of the operator. It would be noticed that the
first impression was of two points, and that each stimulation was so
nearly like the one immediately preceding that no difference could be
noticed. The subject has been led to call a thing two which ordinarily
he would call one, and apparently he loses the distinction between the
sensation of one and the sensation of two. After going through the
procedure just mentioned I put one knob of the æsthesiometer down one
hundred times in succession, and one subject (Mr. Meakin) called it
two seventy-seven times and called it one twenty-three times. Four of
the times that he called it one he expressed doubt about his answer
and said it might be two, but as he was not certain he called it one.