or the other. It is hard to convince the colour-blind of their own
infirmity. I have seen curious instances of this: one was that of a
person by no means unpractised in physical research, who had been
himself tested in matching colours. He gave me his own version of
the result, to the effect that though he might perhaps have fallen a
little short of perfection as judged by over-refined tests, his
colour sense was for all practical purposes quite good. On the other
hand, the operator assured me that when he had toned the intensities
of a pure red and a pure green in a certain proportion, the person
ceased to be able to distinguish between them! Colour blindness is
often very difficult to detect, because the test hues and tints may
be discriminated by other means than by the normal colour sense.
Ordinary pigments are never pure, and the test colours may be
distinguished by those of their adventitious hues to which the
partly colour-blind man may be sensitive. We do not suspect
ourselves to be yellow-blind by candle light, because we enjoy
pictures in the evening nearly or perhaps quite as much as in the day
time; yet we may observe that a yellow primrose laid on the white
table-cloth wholly loses its colour by candle light, and becomes as
white as a snowdrop.

In the inquiries I made on the hereditary transmission of capacity,
I was often amused by the naïve remark of men who had easily
distanced their competitors, that they ascribed their success to
their own exertions. They little recognised how much they owed to
their natural gifts of exceptional capacity and energy on the one
hand, and of exceptional love for their special work on the other.

In future chapters I shall give accounts of persons who have unusual
mental characteristics as regards imagery, visualised numerals,