OR COLOUR BLINDNESS. 181 
to know them by their form*. It will be seen that I have no 
perception of green, which to the majority of observers is the 
predominant colour in nature. It appears to me always red in 
daylight, and some of its varieties seem to me blue in artificial 
light. 
This anomaly dates from my earliest childhood, and has been 
transmitted to me by several of my ancestors. 
IV. 
Examination of the Explanations of Daltonism. 
Several opinions have been put forth, especially in England, 
to account for daltonism. I purpose to indicate them here in a 
succinct manner, accompanying them with some critical remarks 
on their value in the present state of our knowledge. 
Mr. Dugald Stewart, after having noticed in his ‘ Philosophy 
of the Human Mind’+ that there exist differences among men 
in the perception of colours, considers that the cause of it exists 
much more frequently in a weakness or in an error of conception 
than in a defect of the organ. This view of the subject accords 
with the hesitation which some daltonians have to name an iso- 
lated colour, although they have characterized it by comparison. 
There might also be urged in its favour that diversity which 
we have seen prevail between them when called upon to name 
the same colour{. But it seems to me difficult to reconcile with 
this circumstance the fact that many individuals affected with 
daltonism have had a taste for painting, and that the one whose 
history I have sketched had not from his infancy the same in- 
capacity for judging of colours. Moreover, the modest Scotch 
philosopher adds, that in giving this conjecture he is far from 
pretending that there may not be cases in which the affection is 
produced by the alteration of the eye. It may be that the sen- 
* A daltonian in the same way painted a fir-tree in the midst of a landscape 
of a beautiful red. 
+ The first edition is that of 1792, the second that of 1802. See P. Prevost, 
Essais de Philosophie, tome i., p. 249. The illustrious philosopher of Edin- 
burgh was insensible to the least refrangible colours of the spectrum, and could 
not distinguish a red fruit from the green leaves of the tree. (Chambers’ Edinb. 
Journ. vol. iv. No. 171. Brewster’s Treatise on Optics, p. 311.) 
t Dr. Butter of Plymouth, also concluded from the examination of young 
Tucker, that daltonism has a physiological and not an optical cause, since all 
the other functions of the eye preserve their exercise entire. (Edinb. Phil. 
Journal, tome vi. p. 141.) 
