OR COLOUR BLINDNESS. 171 
than is generally imagined. Professor Seebeck found five out 
of forty youths who composed the two upper classes in a gym- 
nasium at Berlin. Professor Pierre Prevost declared that they 
amount to one in twenty. I do not think that this estimate 
is much exaggerated, for I have readily found a number of per- 
sons affected with this singular anomaly. It is necessary to 
remark that daltonians often are themselves ignorant of the 
imperfection of their vision. We may add, however, that we do 
not find in ancient authors any passage which can be referred to 
daltonism, and that the numerous travellers who have gone over 
the old and the new world are quite as little explicit in this re- 
spect. Are we to conclude from hence that this imperfection is 
the lot of modern European nations? It is infinitely more pro- 
bable that in other regions and in past ages the spirit of observa- 
tion less mature has not discovered it. 
Characteristic Signs.—Is there any means of deciding by sim- 
ple inspection of the visual organ of any one, whether he is or 
is not a daltonian? This question is very important, for it is 
not rare to meet with persons devoted to painting or to other 
professions from which the state of their perception of colours 
should have excluded them. I would not venture to affirm that 
the answer should be zn ail cases negative. I have observed, in 
fact, that the daltonians whose eyes are brown, of the colour 
which the English call hazel (noisette), present, under an inci- 
dence more or less oblique, a golden lustre of a peculiar tint. 
Dr. Nicholl has pointed this out in the cases of the young child 
and of the oldest man that have been related above, and Dr. Col- 
quhoun mentions it in the eye of the gardener of Clydesdale. 
Miss Sedgwick says of M. de Sismondi, that he had brilliant 
hazel eyes* ; the illustrious historian was affected with daltonism. 
Struck with this lustre in the eye of a young student of the age 
of fifteen, I subjected him to some trials, which showed, accord- 
ing to my prediction, his inaptitude, until then undiscovered, for 
distinguishing different colours. The eyes of M. D***, of which 
I shall have to speak hereafter, presented the same peculiarity. 
I am, however, far from attributing much importance to this 
remark. 
It has been asserted that there are more daltonians with blue 
than with black eyes. This assertion cannot be sustained ; 
amongst the persons whom I have examined, the great majority 
belonged to the second category. On reckoning up all the ob- 
* Letters from Abroad to Kindred at Home, vol. i. p. 250. 
