four; although it did not at that time excite
much attention. More than a century
afterwards Dalton discovered his incapacity to
distinguish colours; and then, from the
consideration which it received at his hands, the
matter was noised abroad. The Academy of
Geneva attached most incorrectly the term
Daltonian to all who laboured under a similar
defect. It would have been equally rational to
have called every one after Cromwell who
rejoiced in scars, or to have made Whitefield a
synonyme for squinting. Contemporary with
Dalton, Dugald Stewart and Sismondi were
both subject to this peculiarity of vision.
The topic was lost sight of, however, by the
scientific world until eighteen hundred and
twenty-six, when an elaborate paper appeared
by Professor Wartman of Lausanne, which
was translated and published in Taylor's
Scientific Memoirs, Dr. George Wilson, of
Edinburgh, the biographer of Cavendish,
himself a chemist, has recently revived the
discussion. He had his attention first
directed to it by the blunders made by some of
his chemical pupils in reference to the colours
of compounds. Although to a normal eye
very marked changes were soon to occur when
acids or alkalis acted upon vegetable colouring
matter, yet to some students no differerence
was perceptible. One intelligent pupil
constantly erred in deciding on the colours
of precipitates; and Dr. Wilson was led to
investigate his case, which soon proved to be
one of colour-blindness. Further researches
were made and the disease was found to show
itself in three ways.

1. Inability to discern any colour, so that
black and white or light and shade are the
only variations of tint perceived.

2. Inability to discriminate between the
nicer shades of composite colours; such as
browns, greys, and neutral tints.

S. Inability to distinguish between the
primary colours, red, blue and yellow, or
between these and the secondary and tertiary
colours, such as green, purple, orange and
brown.

Total colour-blindness is very rare; but
several well-marked instances were
discovered. One was that of a house-painter
now in Australia. He could not distinguish
any colours but black and white. The
explanation of his prosecuting a calling for
which he was so unfitted is, he was an excellent
draughtsman, with a good eye for form
and great skill in designing. He trusted to
his wife to keep him right in selecting and
mixing colours; but, on one occasion when
she was out of the way and workmen were
scarce, he helped to paint a public building.
He mixed the colours himself, and believed
that he had produced a stone-tint, with which
he proceeded to cover the walls; but, after
he had gone over some square yards, he was
informed that he was painting the building a
conspicuous blue.

Dr. Wilson goes so far as to assert his
belief that the second kind of colour-blindness
is apparently the rule, not the exception,
amongst male persons in this country. This,
he says, arises from the sense of colour being
too little cultivated. Many men hesitate to
pronounce between scarlet and crimson, and
often declare all their shades to be red. If
difficulty in naming be accepted as implying
difficulty in distinguishing colours, it is
inferred that the true perception of them is
a rare gift. The examiner of the chemical
class of the Edinburgh Veterinary College,
numbering about sixty persons, observed
that the great majority declined to give
names to any colours except red, blue,
yellow, green, and brown. Purple and orange
they would not name, although they described
the relation of these to red, blue, and yellow
with accuracy. The difference between pink
and pale blue is a puzzle even to persons
who do not otherwise confound colours.
Thus instances are adduced of three dyers
who constantly commit mistakes with these
tints; of a draper who can match all colours
except drabs; of a professor of chemistry,
who is never sure of the difierence between
blue and green; and of others who are
equally at a loss to distinguish pink from
pale yellow. These, however, are all
differences of degree.

The most important variety of the affection
belongs to the third class. It comprehends
those who mistake red, blue, yellow, purple,
orange, green and brown for other colours, or
who confound all these colours together. In
extreme cases, although some colours are at
times correctly named, there is no certainty
as to any one of them—in milder instances
the majority of these colours are seen; but
two (red and green) and frequently four (the
two last and olive and brown) are not often
distinguished from each other. It is singular
that yellow, which is thought to be one of the
most critical of the primary colours, is, in
reality, that with which the colour-blind have
least difficulty. Blue is also well seen, but the
combination of blue and yellow—green—is
one of the most perplexing in the whole
prism; being often mistaken for blue, yellow,
or even red. Red is still more distracting;
some do not see it at all, others mistake it for
green; and in one case it was confounded
with black. Sufferers under this third class
of visual deficiency are extremely numerous.
Thus, a soldier may have risen through many
grades of the service without ever knowing
under what colours he fought; and a sailor
may have obeyed signals which his better-
sighted messmate read off like print, without
being able to distinguish one flag from
another. The defect is thought to exist amongst
dyers, painters, weavers, clothiers, and others,
whose calling involves familiarity with colour.
It was at one time an object of curiosity to
discover the fate of diseased giants, and
a similar interest may be excited for the
destiny of colour-blind haberdashers and