THE ESSEX FIELD CLUB. 187
rays ; so that it is customary to speak of them as being "colour-blind.'' Typical
cases are recorded, as those of Dr. John Dalton, of Manchester, the founder of
the atomic theory in chemistry ; Professor William Pole, F.R.S., the eminent
engineer, who, as a scientific man, took pains to compare his colour sensations
with other persons possessing normal sight, and described his own case in a most
instructive paper communicated, April, 1859, to the Royal Society.
The speaker produced his own certificate : "Colour Vision, Normal"—given
him after going through the tests applied in the Anthropometric Laboratory, at
the recent meeting of the British Association in Nottingham. This, together
with the fact that for twenty years he had been engaged in a colour factory
without challenge of his capability of matching tints, proved his capacity for
undertaking the examination of others, and for many years he had been in the
habit of so doing when opportunities were presented. He knew a score or more
of well-pronounced cases amongst his male friends, but confessed that hitherto he
had failed in finding a single instance of colour blindness in woman, and it was
here that he desired the aid of his lady friends to search for this defect amongst
their acquaintances. The published statistics proved its rarity—only about four
in a thousand—but he would like to hear of a genuine case, and said that no
names need be mentioned.
Railway guards, engine drivers, firemen and signalmen were now periodically
examined, and no officer in the Royal Navy or Mercantile Marine was appointed
or promoted until he had satisfactorily undergone the imposed tests. The nature
of these tests was then described :—Holmgren's wool test applied in two ways :
First, the patient is requested to select out, say, all the greens from a mixed pile
of coloured hanks ; then to match one or more samples given as patterns. Rail-
way and nautical men were tried with lanterns and shifting glasses at various
distances. Dr. George Wilson, of Edinburgh, whose treatise on "Colour-
Blindness," 1855, first awakened public attention to the necessity for such tests,
used a multi-coloured fan or bundle of dyed plumes. Clerk Maxwell's colour
top had been used to measure the extent of defects. Dr. Jeaffreson's rotating
disc was used to match an indicated sample ; the polariscope and spectroscope
were sometimes employed, and other expedients enumerated in the Royal
Society's 1892 "Report of the Committee on Colour-Vision." It was not
always safe to rely upon a system of naming colours, for everybody did
not know terra cotta, turquoise, purple, russet, or lavender, nor the best
of us where blue ends and violet begins. The lecturer found a pair
of silk tassels, one of iron grey and the other bright green, very useful as a
preliminary test. Most colour-blind persons hesitated immediately, seeing no
difference. The iron grey was sometimes called "crimson," and Professor Pole's
experience helped us with an explanation, for he always saw the extreme red, or
crimson, band of the solar spectrum as a neutral grey. Dr. J. H. Gladstone had
published the fact that he could not distinguish certain shades of blue and green.
The speaker knew a person to whom red and black were alike ; another who
could not see berries on the holly, or any difference, even between a grass lawn
and gravel path ; a bookbinder, who often had bound in violet to match a series
in brown ; an artist, who was obliged to have his paints mixed for him, or take
particular notice of the labels on his colour capsules. The late J. R. Herbert,
R.A., as well as Mulready, were known to be colour-blind late in life, probably by
the lenses or humours of the eye becoming yellow by age, and misleading them
on to excessive indulgence in blue. Instances might be multiplied ; but the fact