SMALL DIFFERENCES OF SENSATION. 



77 



double cliauge this dirterence in the iiinomit of seI^s;lti()ll caiiscd by an increase and decrease of 

 pressure attects every experiment alike, and the liability to error is constant.* 



Throughout our observations we noted the degree ofeonfidenee witii w hicli the ob.server gave 

 liis judgment upon a scale of four degrees, as follows: 



denoted absence of any preference for one answer over its opposite, so that it seemed uou 



sensical to answer at all. 



1 denoted a distinct leaning to one alternati\e. 



2 denoted some little confidence of being right. 



'3 denoted as strong a contidence as one would have about such sensations. 

 We do not mean to say that when zero was the recorded confldence, there was absolutely no 

 sensation of preference for the answer given. We only mean that there was no sensation that 

 the observer noticed when attending to hjs feelings of this sort as closely as he conveniently 

 could, namely, closely enough to mark them on this scale. The scale of confidence Hnctuated 

 cousiderablj'. Thus, when iMr. Jastrow passed from experiments upon differences of weight of 0(1, 

 30, and 15 on the thousand to differences of 20, 10, and 5 on the thousand, although the accuracy of 

 his judgments was decidedly improved, his contidence fell oil' very greatly, owing to his no longer 

 having the sensation i)roduced by a difference of (jO present to his memory. The estimations of 

 contidence were also rough, and might be improved in future work. The average marks seem to 

 conform to the formula — 



P 



»(=clog 



-P 



where m denotes the degree of confidence on the scale, /> denotes tlie probability of the answer 

 being right, iuid c is a constant which may be called the index of confidence. 



To show that this (orniula approximates to the truth, we compare it with the average marks 

 assigned to estimates of differences for which more than a hundred e.ijperiments were made. Mr. 

 Jastrow's experiments are separated into groups, which will be explained below. 



First (iroiip. 



• Tlie number of errors, when au increase of weight was followed by a decrease, was slightly le.ss than wben.the 

 tirst chauge was a decre.ase of pressure. 



