1897,] Psychology. 821 . 
much time to the determination of this distance for different spots on 
the skin, for he found that the two points must be farther apart, to be 
felt as two, on some spots than on others. On the bicepts muscle of 
the upper arm, it is 66 mm. ; on the volar side of the forearm, 40 mm. ; 
on the tips of the index fingers less than 2 mm., etc. Many questions 
arose and many investigators have busied themselves with them; but a 
number of questions in this field, in spite of the numerous books and 
articles on the subject, have in some cases received no attention, and 
in others have not been answered. With the exception of one article, 
the question as to the threshold for the perception of spatial difference 
in the case of two successive stimuli has never been raised. In experi- 
ments with two simultaneously stimulating points, it has long been 
known that the distance which two points must be apart in order to be 
perceived as two at any one spot of skin can be reduced in a very 
marked degree by practice. It was further noticed by Volkmann and 
Fechner that when this distance is reduced by practice on any one 
spot, the threshold for the symmetrically opposite spot on the other 
side of the body undergoes a like reduction without being practiced. 
These investigators gave a purely physiological explanation of the 
phenomenon, viz., that the centre in which the two sets of fibres 
(those from the symmetrical spots) meet is the seat of the change which 
causes the reduction. Their experiment was not, however, so planned 
as to test the question whether a similar reduction of this threshold 
occurs over the entire body. 3 
Our first duty was to undertake a series of such experiments. These 
were carried out by the writer at the Leipzig Psychological Labora- 
tory. As a result, it proved to be true that the same reduction does 
occur over the entire body whenever it occurs on any part of the body, 
and this`result points directly to the inference that the whole phe- 
nomenon demands a central explanation based upon central psychic 
processes. But in connection with these experiments several singular 
phenomena came to light. (1) Not all subjects showed the reduction 
of the threshold by practice: in fact, it occured only in the cases of 
those who knew beforehand what the problem was and what results 
had been hitherto reached by others. In cases where the subject did 
not know these facts and did not surmise them from the nature of the 
experiments, no reduction whatever occurred. (2) In all cases where 
the reduction occurred, there appeared as one result of the practice an 
increase in the frequency of the illusion called by the Germans Verir- 
fehler, i, e., where the subject senses two points when touched by but 
one. In cases in which the series began without these illusions, as 
