galton.org 251
Test Weights
251
We are so accustomed to deal with concurrent impressions that it is exceedingly
difficult, even with the best intention of good faith, to ignore the influence of any
corroborative impression that may be present. It is therefore right to take precautions
against this possible cause of inaccuracy. The most perfect way would be to drop the
weights, each in a little bag or sheath of light material, so that the operatee could not see
the weights, while the ratio between the weights would not be sensibly changed by the
additional weight of the bags. I keep little bags for this purpose, inside the box that holds
the weights.
Arrangement of the Weights.—The
weights are placed in sets of threes, each set in a
separate shallow tray, and the trays lie in two rows in a box. Each tray bears the register-
marks of each of the weights it contains. It is also marked boldly with a Roman numeral
showing the difference between the register-marks of the adjacent weights. This difference
indicates the grade of sensitivity that the weights in the tray are designed to test.
Thus the tray containing the weights WR
0
, WR³,
WR
6
, III. is marked as in Fig. 1, and that which
contains WR²,WR
7
, WR
12
is marked as in Fig. 2
The following is the arrangement of the trays in
the box.
The triplets they contain suffice for ordinary purposes.
Just Perceptible
Ratio
Grade of
Sensitivity
Sequences
of Weights
Just perceptible
Ratio
Grade of
Sensitivity
Sequences
of Weights
1.020
I.
1, 2, 3
1.030
I. ½
2, 3½, 5
1.040
II.
3, 5, 7
1.050
II.½
2, 4½ 7
1.061
III.
0, 3,6
1.071
III.½
0, 3½ 7
1.082
IV.
1, 5, 9
1.082
IV.½
0, 4½, 9
1.104
V.
2, 5, 7
1.127
VI.
0, 6, 12
But it will be observed that sequences of can also be obtained, and again, that it is easy
to select doublets of weights for coarser tests, up to a maximum difference of XII., which
may be useful in cases of morbidly diminished sensitivity.
Manipulation.— A tray is taken out, the three weights that it contains are shuffled by
the operator, who then passes them on
III.
0, 3, 6.
Fig. 1.
V.
2, 7, 12.
Fig 2.
