SMALL Dil'KEKKNCKa OF SE:NttATlOiJ. 71) 



of if.s wci^lit ii|)(>ii llic liii.ucr. Tlii' (litri'iciitiiil pressure was ])i(){luc<'(l by lowerinj;' upon llie pan 

 of the balanee a smaller pan into wliieli tlie proper \veij;hts eould he (inuly fixed; this litth' |)au 

 hiid its bottom of cork, and was placed upon a piece of Hatinel which <!onstaiitly remained in the 

 ])an of tiie bahuu'e. It was lifted oft' and on by means of a fine Indiaruliber threatl. whieli was so 

 mueli stretched by the weight as certainly to avoid any noise dv Jar fiom the momentum of the 

 descending pan. A sufficient weight eonld also be hung on I lie beam of the balance, so as to take 

 oH" the entire pressure from the Hnger at the end of each e.\i)erimeut. This weight could be 

 applied ov removed by means of a cam acting upon a lever; and its bearings ui)on the beam were 

 guarded by India-rubber. It was found that the use of this arrangement, which removed all 

 annoying irregularities of sensation connected with the removal and replacement of the greater 

 (initial) i)ressure, rendered the results more uniform and diminished the probable error. It also 

 shortened the time necessary for performing the experiments, so that a series of 2") experiments 

 was concluded before the effects of fatigue were noticeable. It may be mentioned tliat certain 

 causes tended to the constant decrease of the probable eiTor as the experiments went on, these 

 mainly being an increased skill on the part of the operator ami an eilucafion of the sensibility of 

 the subject. The finger was supported in such a way as to be lightly but firmly held in jiosition, 

 all the muscles of the arm being relaxed; and the India-rubber top of the brass enlargement at 

 the end of the beam of the balance was ne\(U- actually separated from the hnger. The i)rojecting 

 arm of a filter-stand (the height of which could be adjusted) with some attachments not necessary 

 to detail, gently prevented the finger from moving upwards under the pressure exerted by the 

 weight in the i)aii. In the case of Mr. Peirce as subject (it may be noted that Mi'. Peirce is left- 

 handed, while Mr. .Jastrow is strongly right-handed) the tii> of foiefinger, and in the case of Mr. 

 Jastrow of the middle finger, of the left hand were used. In addition, a screen served to prevent 

 the subject from having any indications whatever of the movements of the operator. It is hardly 

 necessary to say that we were fully on our guard against unconsciously received indications. 



The observations were conducted in the following manner: At each sitting three ditt'erential 

 weights wei-e employed. At first we always began and ended with the heaviest, but at a later 

 period the plan was to begin on alternate days with the lightest and heaviest. When we began 

 with the heaviest 2i) observations* were made with that; then 2.5 with the middle one, and then 

 25 with the lightest; this constituted one-half of the sitting. It was completed by three more sets 

 of 2o, the order of the weights being reversed. When we began with the lightest the lieaviest 

 was used for the third and fourth sets. In this way 150 experiments on each of us were taken at 

 one sitting of two hours. 



A pack of 25 cards were taken, 12 red and 13 black, or vice vermi, so that in the 50 experiments 

 made at one sitting with a given ditt'erential weight, 25 red and 25 black cards should be used. 

 These cards were cut exactly square and their corners were distinguished by holes punched in 

 them so as to indicate the scale of numbers (0, 1, 2,3) used to designate the degree of confidence 

 of the judgment. The backs of the.se cards were distinguished from their face^. They were, in 

 fact, made of ordinary playing-cards. At the beginning of a set of 25, the i)ack was well shuffled, 

 and, the operator and subject having taken their places, the operator was governed by the color 



Third and fonitli (/I'diijis. 

 [Marlis 2 mid '■> do not oecur.] 



KiUio of wtriiilits. Sliirlv 0. Mark 1. 



f.UO; 



S '.iW right' :!-i right 



UIM light 2 liglit 



'M''i wKiiig f wroii' 



'•"^•^ I l'J2 wi-oiig :;0 wrong 



1 A.,„ S :i'J^> 'iglit IW right 



^•"~ ;( l:n wrong (i wrong 



At tirst a short pause was made in the set of 2.'), at the optiou oi the subject ; later this was ilisjiiustil « ilh. 



