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C.—AN APPARATUS FOR TESTING THE DELICACY WITH

WHICH WEIGHTS CAN BE DISCRIMINATED BY HANDLING

[Read at the Anthropological Institute, Nov. 14, 1882.]

I submit a simple apparatus that I have designed to measure the delicacy of the

sensitivity of different persons, as shown by their skill in discriminating weights, identical

in size, form, and colour, but different in specific gravity. Its interest lies in the accordance

of the successive test values with the successive graduations of a true scale of sensitivity,

in the ease with which the tests are applied, and the fact that the same principle can be

made use of in testing the delicacy of smell and taste.

I use test-weights that mount in a series of “just perceptible differences” to an

imaginary person of extreme delicacy of perception, their values being calculated

according to Weber’s law. The lowest weight is heavy enough to give a decided sense of

weight to the hand when handling it, and the heaviest weight can be handled without any

sense of fatigue. They therefore conform with close approximation to a geometric series;

, WR¹, WR², WR³, etc.,

and they bear as register-marks the values of the

successive indices, 0, 1, 2, 3, etc. It follows that if a person can just distinguish between

any particular pair of weights, he can also just distinguish between any other pair of

weights whose register-marks differ by the same amount. Example: suppose A can just

distinguish between the weights bearing the register-marks 2 and 4, then it follows from

the construction of the apparatus