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POPULAR SCIENCE REVIEW. 
colour-blind persons arrange different shades of the same 
colour according to their intensity, in a series which did not 
satisfy their eyes ; and further, that their arrangement of dif- 
ferent colours according to their intensities seemed discordant 
to both these gentlemen. 
The celebrated Dugald Stewart, and Dr. Darwin, the poet 
and botanist, could only by shape discover the difference 
between cherries and the leaves among which they grow. 
Dr. Dalton, the propounder of the atomic theory in chemistry, 
was not convinced he was colour-blind, until by accident 
observing the colour of the flower of the Geranium zonale by 
candlelight in the autumn of 1792. The flower was pink, 
but it appeared to him almost a sky-blue by day : in candle- 
light, however, it was astonishingly changed, not having then 
any blue in it, but being what he called red ; forming a 
striking contrast to the blue. He also compared sealing-wax 
to one side of a laurel leaf, and a red wafer to the other, and 
his doctor’s scarlet gown to the leaves of trees. “I have seen 
specimens,” writes Dr. Dalton, “ of crimson, claret, and mud 
which were very nearly alike. Crimson has a grave appear- 
ance, being the reverse of every showy or splendid colour. 
The colour of a florid complexion appears to me that of a dull, 
opaque, blackish blue upon a white ground. Diluted black 
ink upon white paper gives a colour much resembling that of a 
florid complexion. It has no resemblance to the colour of blood.” 
From the care with which Dr. Dalton investigated his own defect, 
it has become popularly known as “ Daltonism.” Nor was his 
case at all peculiar with regard to flowers, for the colour-blind are 
constantly found unable to distinguish the petals of the scarlet 
geranium from its leaves, the flowers of the wild poppy from 
the unripe corn amongst which it is growing. Moreover, those 
who thus mistake scarlet, regard green as a darkish colour, 
and confound it with drab. 
The number of cases now upon record of persons afflicted 
in this way are very considerable ; though until within these 
late years it was supposed to be confined to a very few 
individuals. From the calculations of various authors, that 
one person out of every fifteen is colour-blind, and from the 
investigations of the late Dr. Wilson upon 1,154 persons at 
Edinburgh made in 1852-53, we gather that — 
1 in 55 confounded red with green, 
1 in 60 confounded brown with green, 
1 in 46 confounded blue with green ; 
lienee, that one in nearly every eighteen had this imperfection. 
Professor Siebeclc found five out of forty youths in the two 
upper classes in a school at Berlin colour-blind. Professor 
