614 PSYCHOLOGY. 



of an electric spark, when their interval was made as small 



as about -^-l^ of a second.* 



A\ ith the eye, perception is less delicate. Tavo sparks, 

 made to fall beside each other in rapid succession on the 

 centre of the retina, ceased to be recognized as successive by 

 Exner Avhen their interval fell below COM".! 



Where, as here, the succeeding impressions are only two 

 in number, we can easiest perceive the interval between 

 them. President Hall, who exjjerimented with a modified 

 Savart's wheel, which gave clicks in varying number and at 

 varying intervals, says : % 



"In order that their discontinuity may be clearly perceived, four or 

 even three clicks or beats must be farther apart than two need to be. 

 "When two are easily distinguished, thi'ee or four separated by the same 

 interval . . . are often confidently pronounced to be two or three 

 respectively. It would be well if observations were so directed as to 

 ascertain, at least up to ten or twenty, the increase [of interval] re- 

 quired by each additional click in a series for the sense of discontinuity 

 to remain constant throughout." § 



* The exact interval of the sparks was 0.00205 '. The doubleness of 

 their suap was usually replaced by a single-seeming sound when it fell to 

 0.00198", the sound becoming louder when the sparks seemed simultaneous. 

 The difference between these two intervals is only x^tJWtf of a second; and, 

 as Exner remarks, our ear and brain must be wonderfully eflScient organs 

 to get distinct feelings from so slight an objective difference as this. See 

 PUiiger's Archiv, Bd. xi. 



f Ibid. p. 407. When the sparks fell so close together that their irradi- 

 ation-circles overlapped, they appeared like one spark moving from the posi- 

 tion of the first to that of the second; and they might then follow each 

 other as close as 0.015" without the direction of the movement ceasing to be 

 clear. When one spark fell on the centre, the other on the margin, of the 

 retina, the time- interval foi successive apprehension had to be raised to 

 0.076"- 



X Hall and Jastrow : Studies of Rhythm, Mind, xi. 58. 



i^ Nevertheless, multitudinous impressions may be felt as discontinuous, 

 though separated by excessively minute intervals of time. Grunhagen 

 says (Pfluger's Archiv, vi. 175) that 10,000 electric shocks a second are felt 

 as interrupted, by the tongue (I). Von Wittich {ihid. xx. 329), that between 

 1000 and 2000 strokes a second are felt as discrete by the finger. W. 

 Preyer, on the other Land (Die Grenzen des Empfindungsvermogens, etc., 

 1868, p. 15), makes contacts appear continuous to the finger when 36.8 ot 

 them follow in u, second. Similarly, Mach (Wiener Sitzgsb., Li. 2, 142) 

 gives about 36. Lalanne (Comptes Rendus, Lxxxir. p. 1314) found summa- 

 tion of finger contacts after 22 repetitions in a second. Such discrepan, 

 figures are of doubtful worth. On the retina 20 to 30 impressions a second 



