Bauhaus-Universität Weimar

Experimental Psychology: A Manual of Laboratory Practice, Vol. II: Quantitative Experiments, part 1: Student's Manual
Titchener, Edward B.
§ 16. The Method of Limits 
The photometric value of the brightnesses employed should be 
determined by the Kirschmann photometer (set up in the same 
position as the mixers) both at the beginning and at the end of 
the experiment. Even if the discs are carefully kept in a dark 
drawer in the intervals between the laboratory sessions, there is 
danger of fading. The two sets of photometric values are aver¬ 
aged for the final determination of the DL. 
The results are, of course, entered fully in the note-book in terms of de¬ 
grees. At the end of the experiment, however, the results of the various 
series must be translated into photometric values. Suppose, e.g., that a 
black and a white are used, whose brightness-ratio is given by the photome¬ 
ter as i : 40. Then the1 value ’ of a disc of 180° B and a 1800 W is 180 X 
1 + 180X40 or 7380 ‘photometric units.’ If the amount of the average 
DL were 50, the value of the corresponding disc would be 175 X 1 + 185 X 40, 
or 7575 units. The relative DL would then be expressed by the fraction 
195 i 
—— or —. 
7380 38 
Preliminary experiments are to be made before the experiment 
proper is begun (p. 60 above). But more than this : E should 
always give 0 a little practice at the beginning of each session, 
before entering on the series laid out for the day ; he should on 
no account start the series out of hand. The time elapsing 
between session and session, even if it is no more than 24 hours, 
is long enough for 0 to get ‘ rusty ’ ; only after a practice series 
does he warm up to the work, as one says, or get into the swing 
of the experiments. This rule must never be broken. 
As soon as the conditions of the experiment have been 
arranged, E makes out a plan or 'Table in which all the con¬ 
stants of the apparatus (distances, heights, etc.) are accurately 
entered. The plan is rigorously adhered to in the successive 
Determination of the DL for Tone.—This experiment is best 
performed by the aid of tuning forks. Two arrangements are 
in general use. 
(1) Materials.—Two forks, the one of which is furnished 
with riding weights. Piano hammer. Felt. Stop-watch.


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