The measurement of visual sensations

The paper is a criticism of Dr L. F. Richardson's proposed method of measuring sensations by mental estimates. In the absence of agreed principles the discussion is conducted in the light of an analogy with thermometry. It is argued (1) that Dr Richardson's own facts prove that the method of mental estimates is inferior to another available method of measuring the same thing. If his facts and his implied propositions are true, indirect measurement by means of his S must be preferable to direct measurement by R. (2) That any arguments that Dr Richardson might base upon the fact that his method of measurement leads to a numerical law between R and S would be valid only if he proved that this law is true and not empirical. (3) That if his method measures anything, that something is almost certainly not a sensation; at least, that he has provided no evidence that it is a sensation. Finally it is pointed out that Dr Richardson's method indicates the theoretical possibility of a method of measurement based upon ordering differences of increasing degree; but there is no evidence that the method is ever practically possible.