Bauhaus-Universität Weimar

Experimental Psychology: A Manual of Laboratory Practice, Vol. II: Quantitative Experiments, part 1: Student's Manual
Titchener, Edward B.
§ 13- Webers Law 
tion of the shot. E places the shot in the forceps ; rests the jaws of the 
forceps on the card held horizontally ; releases the shot, so that it settles 
down upon the card ; and lets the jaws of the forceps close again. He ad¬ 
justs in this way for every test. An extra forceps is useful, for picking up 
the fallen shot. 
Some little practice is required in the manipulation of the forceps. The 
pinch of thumb and finger must be firm and uniform ; the downward pres¬ 
sure must not be so strong that the forceps scrapes against the screw-head. 
Method.—The strict serial method of Exp. I., or the haphaz¬ 
ard arrangement of Exp. IL, may be followed. The unit of 
change must be determined from the preliminary experiments ; 
it is probable that a step of 0.5 mm. will be small enough. 
Questions.—O (1) Sometimes, in the course of these experi¬ 
ments, you are quite sure that you hear the sound ; sometimes, 
you say with confidence that you only ‘ imagine ’ it. How do you 
distinguish between a real and an imagined sound ? 
0 (2) Have you had any similar experience in preceding ex¬ 
periments ? Is it present to the same degree in all ? Why ? 
E and O (3) Can you suggest experiments that would make 
it difficult for you to decide whether or not an objective stimulus 
had been given, and that would therefore bring out clearly the 
criteria which you apply in distinguishing between a real and an 
imagined impression ? 
§ 13. Weber’s Law.—We said above (p. xxxiv) that the formula 
of correlation between A’-magnitude and sense-distance has, in 
certain sense departments, been worked out with some degree of 
fullness. We said, also, that the formula has been worked out 
in terms of two different kinds of sense-distances : in terms of 
supraliminal distances, arbitrary distance-units that are larger 
than the DL ; and also in terms of liminal distances, i. e., of the 
DL or the j. n. d. itself (p.xxxv). In the former case, it has been 
worked out most successfully for brightnesses ; in the second, it 
has been verified for a large number of different sensations. 
We have now to see what it is and how it is derived. 
(1) Suppose that we have before us a series of 50 grey papers, 
delicately graded from dark to light. We are required to divide 
UP this practically continuous series into 6 equal sense-distances. 
Keeping the first and the last, the darkest and the lightest papers,


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